Exploring the everyday

 

Human beings quickly become accustomed to the world they grow up in. We take it for granted that aircraft fly, some cheese has holes in it, dust is grey, glue sticks, coffee tends to keep you awake, fire burns - and so do your eyes when onions are being chopped - mobile telephones drop out and washing machines seem to like nothing better than to gobble up socks. But why? The world of the everyday offers many fascinating puzzles - and the explanations often cause us to look at things in a different way.

Why can't we do without physics and chemistry in the kitchen ?

It's true that everything we do in the kitchen has something to do with physics or chemistry, from cooking our food to freezing it. This is because rising and falling temperatures, even just mixing and stirring, can cause changes to occur. The molecules that make up foodstuffs, the water or oil they are cooked in and the air in the environment all undergo chemical reactions. These days, even top chefs make use of our better understanding of the physics and chemistry of cookery to improve their creations.

Why doesn't food stick to non-stick pans ?

Whether or not a fried egg sticks to a pan is a question of chemistry. As a rule, the molecules of an uncoated metal frying pan and the molecules of frying food are strongly attracted to each other. This creates bonds between the metal surface and the carbohydrates or proteins in the food, and this chemical attraction becomes visible when the food sticks to the pan and burns. Frying with butter or oil can reduce sticking because these fats provide a temporary intermediate layer. However, the only long-term solution is to use a pan with a permanent layer on its surface, such as Teflon or a similar synthetic material. The compounds that make up fried eggs and other culprits have nowhere to stick on these kinds of surfaces. The non-stick coating is held firmly to the pan because the coating's molecules cling to the countless imperfections on the rough metal surface. However, the molecules don't stick to each other all that well, which is why non stick pans scratch so easily.

Why does egg white stiffen when whisked ?

Egg white is 90% water. It also contains long protein molecules which keep a propter draft was longues loosely connected. In their normal state, the proteins are in the form of dense spheres which can best be compared to balls of wool. When egg white is whisked until it stiffens, two things happen: air bubbles form in the water and protein mixture and the protein molecules unravel. Because they consist of both hydrophilic (water-loving) and hydrophobic (water-repelling) parts, they attach themselves to the boundaries between the water and the air bubbles and so encapsulate the air bubbles - with the result that the egg white becomes stiff.

If the egg white is beaten too much, however, the water can detach itself from the proteins again and the process will fail. Traces of egg yolk can also cause problems. This is because its molecules weaken the coating around the air bubbles and make it harder for the proteins to attach themselves. Egg yolk and other fats only cease to be a problem once the egg whites are stiff.

Why does tomato sauce get stuck in the bottle ?

Tomato sauce is a highly viscous paste made from tomatoes, spices and various other additives. Hence, the sauce can only move if it is shaken or if sufficient force is applied. Everyone knows from pouring water out of a bottle that the liquid that leaves the bottom of a bottle has to be replaced by air bubbling through the water from the open end of the bottle. Tomato sauce, however, is too viscous to allow air bubbles to pass through it easily. This is why a vacuum forms in the bottom of the bottle when the sauce moves away from it. As a result, a pressure difference is created that works against the force of gravity, which is pulling the sauce downwards.

When tomato sauce stubbornly refuses to leave the bottle there are several ways of getting it out. Plastic bottles of sauce are flexible and can be squeezed to eliminate the vacuum. Sauce in rigid bottles, on the other hand, can be helped along by hitting the base of the bottle, sometimes with disastrous results. The third method helps with both types of container- simply insert a drinking straw deep into the bottle so that air can pass through the tomato sauce to the bottom.

Why do jelly babies grow larger in water ?

Jelly babies made with gelatine swell in cold water, while those made with gum arabic, agar, pectin or starch fall apart. Contrary to popular belief, the fact that gelatine jelly babies swell up has nothing to do with osmosis. In osmosis, water passes through a membrane to balance out different concentrations of dissolved particles, such as salt or sugar. However, jelly babies are not wrapped in a membrane. Gelatine is able to absorb large quantities of water because the water molecules attach themselves to particular structures of the protein molecules in the gelatine, greatly increasing their volume. Chemists call this process gelatification.

What gives sparkling water it's sparkle ?

The addition of carbon dioxide is what gives mineral water a refreshing sparkle. This requires very high pressure and relatively cool conditions, for only then will the gaseous carbon dioxide dissolve - forming carbonic acid- and be evenly distributed in the water. In an unopened bottle of mineral water, the pressure is four to five times higher than it is in the ambient air. When a bottle is opened, the pressure is reduced and some of the carbon dioxide escapes. This is visible in the form of bubbles that come fizzing up suddenly. Carbonated mineral water fizzes when pressure in the bottle is reduced, but not all the carbon dioxide bubbles escape at the same time. This is because, although carbon dioxide is lighter than water - which is why the fizz travels upwards - it is heavier than air and slows down when it reaches the air.

In the case of naturally sparkling mineral water, the water absorbs carbon dioxide as it seeps through volcanic rock. However, as the water travels to the surface, the fizz escapes, but the gas is collected and added to the water just before bottling.

How is instant coffee manufactured ?

When we make a cup of instant coffee we are actually brewing it for the second time. This is because the manufacture of instant coffee begins with making coffee. Roasted and ground coffee beans are boiled in pots, just as they would be for mocha or Turkish coffee, only much more intensely. When most of the water has evaporated and the coffee grounds have been removed, the thick coffee solution that remains is dried. There are two ways of doing this: spray-drying and freeze-drying. With spray drying, the wet coffee solution is sprayed through a fine nozzle into a drying tower, from the bottom of which hot, dry air rises. The remaining water evaporates and the instant coffee powder collects at the bottom of the tower. With freeze-drying, the coffee extract is chilled to -40°C in a matter of minutes. The slabs of iced coffee then pass into a warm vacuum, where sublimation takes place - the solid ice turns straight into water vapour, bypassing the liquid stage. Again, all that is left behind is the familiar dry, water-soluble powder.

How do the holes get into cheese ?

Cheese is made by adding rennet - a combination of various enzymes-to milk so that it coagulates. In the process, some of the protein and fat turn to 'curds' which separate from the rest called 'whey'. The curds are then pressed into moulds and stored for a time until they mature.

In some cheeses, such as Tilsit, irregular holes appear because the curds are poured loosely into a mould and not pressed. More whey drains away during storage, leaving cavities in the cheese. Large, spherical holes like those in Emmental-known as 'Swiss' cheese in some countries - develop in cheeses that are packed into moulds and develop a firm rind. The holes are produced by the lactic acid bacteria present in raw, unpasteurised milk. To make cheeses with particularly large holes requires the addition of propionic acid bacteria, which consume the lactose and produce carbon dioxide as the cheese matures. The small bubbles join with larger ones, and as the rind develops and the curds harden, they are unable to escape. The holes vary in size according to how the cheese is matured and the number of bacteria present.

How do baking powder, sourdough and yeast work ?

Baking powder, sourdough and yeast all serve to relax dough and make it rise by producing bubbles of carbon dioxide. Baking powder creates carbon dioxide by means of a chemical reaction, while with sourdough it is produced by bacteria, and with yeast the job is done by the tiny, individual yeast fungi.

Baking powder contains two salts that react with each other. These are usually sodium bicarbonate and an acid salt. The chemical reaction occurs as soon as water and heat are introduced-which is why dough only rises once it is in the oven. However, the volume of yeast dough and sourdough increases prior to baking. Lactic and acetic acid bacteria in sourdough produce carbon dioxide from sugar at temperatures between 20 and 35°C. Single-celled yeast fungi create carbon dioxide when fed with sugar and water and exposed to temperatures of about 32°C. This is why sourdough and yeast dough have to rest in a warm place before baking. The heat of the oven at any temperature above 45°C will kill off the bacteria and yeast fungi.

How are 'best-before' dates determined ?

It is difficult to tell whether packaged food is still usable or has already gone off, why the food industry developed the concept of 'best-before' dates. Best-before dates are often seen as equivalent to 'use-by' dates, but the two terms differ in one important detail. The best before date indicates that the product can be kept at least until the specified date, provided it has been correctly packed, transported and stored. Food manufacturers are therefore obliged to find out the average period after which their products will spoil in sealed packaging, and the best-before is set well before that time. Once passed its best-before date, food does not have to be thrown away immediately. It may continue to be sold and used, provided it has been examined and found not to have spoilt. In contrast, products that have a use-by date have to be removed from circulation immediately that date has passed. This is why particularly perishable foods like meat are given a use-by date. The same applies to the 'expiry' date, which is mainly used on pharmaceutical products.

Why shouldn't olive oil be stored in refrigerator ?

The mono- and polyunsaturated fat content of an edible oil is what determines the temperature at which it will solidify, what its burn point is in the frying pan, and how quickly contact with oxygen will turn it rancid. Olive oil consists of up to four-fifths monounsaturated fatty acids which makes it ideal for frying. The oil keeps best at about 15°C, and in the refrigerator it will turn cloudy with flaky lumps - but only until its temperature rises again. Whether or not this spoils the flavour is a matter of opinion.

Other oils with higher proportions of polyunsaturated fatty acids are more sensitive to heat and oxygen, and unrefined salad oils in particular keep longer under cool conditions. All types of edible oils are best stored in the dark or in dark-coloured bottles.

Why doesn't honey have to be stored in the fridge ?

Honey is widely thought to be the only food that does not spoil. This is because of its high sugar and low water content, which makes it an environment in which bacteria and other micro organisms cannot multiply. Honey also contains certain enzymes and aromatic acids that are effective against microbes. This is why doctors may even use honey to help serious wounds heal more quickly-in the form of honey bandages, for example. On average, honey consists of 70-80% sugar and only 15-20% water. Added to this are small amounts of pollen, flavourings, minerals, vitamins, enzymes and other proteins. Unlike sugar - which is pure sucrose - honey contains various kinds of sugar, especially glucose, fructose and small amounts of sucrose and dextrin. The exact proportions of the various sugars determine how long it will take for a particular honey to crystallise and harden. And even the fridge cannot prevent that from happening.

Why do human beings cook their food ?

Humans could survive on a diet of raw food. However, cooking and smoking food greatly extend the menu available to us because these processes destroy inedible, bitter and toxic substances. Some plants are even indigestible until their constituents are broken down by cooking. Furthermore, heat kills off any germs and smoked foods keep for considerably longer than fresh food does.

Why do we eat- and enjoy it ?

Firstly, of course, food guarantees our continued survival. It provides the body with vital nutrients and supplies us with energy. To enable the body to use food, during the process of digestion various organic compounds - protein, fat and carbo hydrates - are released. These provide the energy that the body uses to move and think, to fuel such automatic physical processes as those involved in breathing and circulating blood and, last but not least, to maintain a constant body temperature. Minerals, vitamins and trace elements are essential for finetuning the metabolism, and the water in food provides the body with a basic supply of liquid.

As a rule, eating satisfies us and makes us feel good. Right from the time when we are breastfed as babies, taking in food is linked to the release of neurotransmitters in the brain which are responsible for pleasure and happiness. In addition, thanks to their unusually high number of nerve endings, the lips function as an erogenous zone which is also stimulated to some extent by eating.

Finally, a full stomach triggers a feeling of wellbeing in the brain. There are various ingredients in food that go one step further-the best-known among these probably being chocolate which directly releases the 'happy hormones' endorphin and serotonin. Wheat beer contains vitamin B, which is considered beneficial to the nervous system while hot and spicy foods work indirectly. As soon as the taste buds feel the heat they emit pain-relieving neurotransmitters which also have a mood-enhancing effect.

What makes us eat - hunger or appetite ?

Hunger does not care what the body gets to eat so long as it gets something. Hunger is mainly triggered by physical signals, including an empty stomach, falling blood sugar levels and metabolic changes. Serotonin and other hormones regulate these signals in the brain's control centre for hunger, situated in the hypothalamus, or interbrain.

Appetite, on the other hand, allows people to eat even when they feel 'full' This desire is in fact, dependent on the food on offer, its colour, the way it is prepared and the way it is perceived by an individual. Appetite, therefore, tends to be psychologically driven, specifically by the limbic system which also controls emotional behaviour and learning processes. Simply thinking about certain foods or their aroma can stimulate the appetite.

The greater the choice and the bigger the portions, the more people will eat-regardless of how hungry they are. They are more likely to tuck into food they know will taste good, and if a favourite dish-such as fried chicken-has been dyed blue it is likely to be left untouched. The same applies to crunchy snacks of insects for the average westerner, although in some cultures they are considered to be a great delicacy.

Will a pickled herring cure a hangover ?

A pickled herring is a flavoured fish, popular in some parts of Europe, that is sometimes said to cure a hangover. It is most effective against headaches and nausea if eaten prior to the consumption of alcohol, but will not cure a hangover. This is because a hangover is caused by the breakdown of alcohol in the body during which water, minerals and vitamins are eliminated from it, which causes dehydration. In addition, alcohols produced in fermentation - which give drinks their particular flavour - degrade into toxic intermediate products that cause the heart to pump less oxygen to the brain. Different types of alcohol produce different levels of cytokines. These are the neurotransmitters that also cause weakness, nausea and headaches. Studies involving several kinds of alcohol have found that brandy can cause worse hangovers than red wine, rum, whisky, white wine, vodka or even pure alcohol.

The best expert advice is to eat high-protein, fatty foods like fish, nuts or cheese prior to consuming alcohol. These top up mineral levels and hold the alcohol in the stomach for longer where it begins to break down. Consuming plenty of water or fruit juice during a night out, and also before going to bed will prevent dehydration, as well as dilute the alcohol and thereby reduce overall consumption. The morning after is best dealt with by eating a breakfast high in minerals and vitamins and taking outdoor exercise. Painkillers tend to do more harm than good when mixed with residual alcohol in the body.

Can it really be healthy to have worms in your stomach ?

In theory, tapeworms in the intestine could be used as a Islimming aid. Beef and pork tapeworms can live inside human beings for up to 25 years, and most infected people will not experience any problems. During the infestation, people lose weight because the worm lives on the remains of food in the intestine, preventing it from being absorbed by the body. Only occasionally does its presence cause abdominal pain, flatulence or a rumbling stomach. Sufferers can be entirely unaware they are harbouring a tapeworm, unless they happen to see parts of it following excretion. Medication will in most cases completely eradicate tapeworms.

However, worm infestation should be taken seriously because there are other kinds of tapeworm, as well as roundworm and threadworm, which can trigger highly unpleasant symptoms. Their larvae can penetrate the lungs and other organs where they can cause serious damage. The healthiest way for worms-highly prized in some African and Asian cultures - to find their way into the intestines is for them to be eaten boiled or fried.

Why does asparagus make urine smell ?

The reason why some people find that their urine smells strange after they have eaten asparagus is sulfur. Asparagus contains the amino acid asparagine which contains sulfur compounds. Ordinarily, this has a mild smell of asparagus but enzymes in the stomach can break down the sulfur compounds. British biochemists have identified six different sulfur compounds in the urine of some people that are responsible for the sometimes disagreeably rancid smell.

However, not everybody 'sulfurises' asparagus. Evidently only one in two Europeans has the necessary enzyme in their stomachs. The predisposition to this characteristic is a dominantly inherited trait, which means that one parent passes it on to all of their children - even if the other parent is able to eat asparagus without smelly consequences. However, the gene that is responsible for this has yet to be identified.

Another British study has discovered that this distinctive scent is only perceived by some people. This means that some people produce the sulfurous smell and don't know anything about it. It seems that only 10% of the population has a sense of smell with the necessary degree of sensitivity.

Why does chopping onions bring tears to your eyes ?

Onions contain sulfur compounds that are released as soon as the knife cuts into the vegetable. The substance that causes the greatest trouble is a compound known as propanthial S-oxide, which rises into the air and combines with water in our tear ducts to form stinging sulfuric acid, which is what makes us cry.

This is why the only way to stop chopping onions being such a tearful business is to prevent all, or at least some, of the oxide from getting into the eyes. This could be done by cutting the onions under water, turning your face away or wearing protective goggles - even wearing contact lenses can help. Also, the more coarsely you chop the onion and the sharper the knife the less oxide is released. It is not clear whether it is also helpful to chill the onion prior to chopping it.

Does smoking help to make you slim ?

Nicotine from cigarettes travels straight to the brain where it links up with nicotinic acetylcholine receptors and sets in motion a whole series of hormonal and physical reactions. These include the dulling of hunger pangs and a reduction in the appetite, and so it is true that smokers do eat less. On a purely practical level as well, the fact that smokers' hands and mouths are engaged in the business of smoking means they are less likely to snack. At the same time, nicotine makes the heart beat faster, raises blood pressure and increases intestinal functioning, so that the body uses more energy. This is why smokers weigh on average 2 to 3 kg less than nonsmokers.

The reason people put on weight when they give up smoking is because smoking releases hormones in the brain's reward centre, known scientifically as the nucleus accumbens. People who try and quit smoking therefore have to look elsewhere for the accustomed feeling of reward or comfort, and many turn to food instead. At the same time, quitting cigarettes improves one's sense of taste and smell, which makes many new nonsmokers enjoy their food more so that they eat larger quantities than before. In most cases, however, they only put on a few kilos which they can lose by finding other distractions and exercising more.

Why do we burp ?

Belching is a way of removing air from the digestive tract that does not belong there. It is especially important that babies are 'burped' because they are not yet able to properly coordinate feeding and breathing. A baby may inadvertently swallow air which can cause unpleasant pressure in its oesophagus and stomach, which results in discomfort and crying.

Adults also take too much air into their stomachs if they eat or drink too quickly, and some people have been known to swallow huge quantities of air.

When we burp after drinking a carbonated drink, such as sparkling mineral water, beer or champagne, the cause may be carbon dioxide, which escapes from the carbonated drink into the stomach. In rare cases, faults in the mechanism that closes the link between the oesophagus and the stomach can lead to belching which may also bring up some stomach contents as well as air.

Apart from ruminants, such as sheep or cattle, most animals cannot belch. The ruminants mainly bring up methane, a gas which is released by bacteria in their digestive tracts.

Why do we feel thirsty ?

Thirst develops in the brain, when the body tells it that there is too little water or too much salt in the blood. In both cases, the cells are deprived of water which, in extreme cases, can disrupt both the intricate processes taking place in individual cells and the overall function of the larger vital organs. When the proportion of water falls by half a percent - this is the thirst threshold - the body sends a message to the brain's thirst centre, where the hypothalamus produces the anti-diuretic hormone- or ADH - which slows down

the excretion of water from the kidneys. The kidneys control the body's fluid and electrolyte balance and can be badly affected by long periods of dehydration, which can leave them unable to function properly. At the same time, the production of saliva in the mouth is reduced - this is why a dry throat is a sign of thirst..

Heavy sweating and fever, diarrhoea and high levels of alcohol consumption quickly lead to dehydration. If a human being loses more than 15 to 20% of his or her body weight in the form of water they will die of thirst.

Why does coffe help to keep us awake ?

The caffeine in coffee and black tea passes into the bloodstream and affects the central nervous system. It competes with the neurotransmitter adenosine, which would ordinarily attach itself to certain receptors in the nerve cells to calm them and prevent them becoming overloaded. Caffeine has a similar structure and fits into the same receptors. It therefore blocks them and prevents the calming adenosine effect, which leads to increased alertness and an ability to concentrate.

However, constant and heavy coffee consumption causes the nerve cells to react by creating more receptors. Therefore, the same amount of coffee will, in the long term, have a less pronounced effect. Suddenly giving up caffeine can cause temporary physical withdrawal symptoms. Coffee and tea have the greatest stimulating effect if they are drunk in small quantities throughout the day.

Why do flags flutter ?

The behaviour of everyday objects can usually be explained with the help of physics. However, even such apparently simple phenomenon as a length of fabric fluttering in the breeze can turn out to be extremely complex and very hard to describe mathematically. What physicists do know for certain is that air swirls down the length of the fabric, turning into chaotic turbulence at its end - where the flag flutters.

Why do kettles sing ?

Kettles emit numerous subtle sounds, ranging from hisses to shrill whistles while water is being heated - the actual sounds depend on water temperature. At the beginning of the heating process, tiny air bubbles burst at a rapid rate. These are dissolved in the water, and then travel upwards. When the water reaches about 65°C, steam bubbles form on the bottom of the hot kettle and collapse in the cooler upper layers of water with a loud, deep, plopping sound. These bubbles don't reach the surface until the water reaches a temperature of almost 100°C, and then the muffled plop is replaced by a gentle bubbling sound. The escaping steam passes through a whistle - if the kettle is fitted with one-which creates the shrill sound that signals that the water is boiling.

Why can't we see the end of a ray of light ?

A 'ray' is an idealised narrow beam of light - a line showing the path of the light. You see a beam of light directed straight at you because virtually all the light reaches your eyes. You can see a beam of light from the side if the light stays on for while and some of the light is scattered off dust or fog in the air.

Light can be described as electromagnetic waves which, unlike sound waves, can travel through a vacuum. The waves travel in straight lines at 300 000 km/sec (in a vacuum) until they are absorbed or deflected.

When you switch a torch on at night you can't see the front of the beam advancing out into the darkness - or the retreating end of a beam when you switch the torch off-because the radiation is moving so rapidly.

Why do mirrors show left and right in reverse, but not top and bottom ?

A flat mirror doesn't reverse anything - it is only the brain of the viewer that misinterprets the image. It places the viewer in the position of the person in the mirror, and suddenly the wedding ring worn on his or her left hand appears on the right. A concave mirror will show a reversed mirror image if the object is correctly located relative to the mirror - at a point beyond the mirror's focal distance.

Why don't clothes look the same in the changing room as they do outside ?

Fitting rooms are lit by incandescent light bulbs or fluorescent tubes, while exterior spaces are lit by the Sun. Although we usually think of all three as white light, they are actually made up of slightly different combinations of wavelengths (colours), although the differences are not obvious unless seen side by side.

Physicists assign each light source a 'colour temperature', while common usage some- times assigns a quite different 'temperature'. We talk of incandescent bulbs creating 'warm' light, while fluorescent light is 'cold' and the light of the midday Sun is 'neutral'. As each of the different combinations of light waves strike clothing, some is reflected and reaches the retina of a viewer's eye. The light spectrum that reaches the eye will therefore be different, depending on the light source. This is especially noticeable on shopping trips, when colour might be especially important. The brain usually balances out slight colour variations. If it 'knows' that a dress is turquoise, it will appear just as turquoise in the midday Sun as it does at sunset.

How does electric current 'flow' ?

Just as water flows through a pipe, electricity can be I thought of as electrically charged particles - usually electrons - flowing though a wire or other conductor. Most electrons - whether they are in air, plastic or metal - are constrained so that they are only ever able to move around the nucleus of the atom of which they are a part. In metals, however, some of the electrons are able to jump easily between the tidily arranged atoms. A battery is a device that tries to drive electrons in one direction (from the negative terminal to the positive terminal, since electrons carry a negative charge). If the battery isn't connected to anything, the electrons end up unevenly distributed, with an excess at one of the battery terminals (the negative one), and a deficiency at the other terminal (the positive one). If the two terminals are then connected to each other by a metal wire, a current will flow. However, the surplus electrons don't just whiz from one end of the wire to the other. Rather, the whole pack of evenly distributed, but loosely held, electrons already in the wire are propelled along its length. They move forward through the wire, so that some of them are pushed out at the other end and back to the battery (doing some work on the way if the battery is part of a device such as a radio). The individual electrons move very slowly during this process, but the signal that the motion has started travels at almost the speed of light.

What is fire made of ?

Fire is a combination of chemical and physical changes in which substances interact to release heat, light, smoke and ash. Whether it is a fire in electrical insulation, an oil fire or a simple domestic fire, they all require three ingredients - combustible material, oxygen and a thermal trigger. If one of these is absent, the fire goes out. This principle is the basis of fire-fighting, which aims to cool the flames by smothering them, removing oxygen, or depriving them of fuel.

Combustible material often consists mainly of carbon and hydrogen and can be almost anything solid, liquid or gaseous, from hydrogen to wax, oil, wood and plastic. Even metal will burn provided the temperature is high enough. Sufficient quantities of oxygen are usually supplied by the air, and the energy to light a fire is produced by friction or sparks.

Do car tyres always have to be black ?

The black colour of tyres was originally the result of the the production methods employed. Rubber - which is a pale yellow, milky substance obtained from rubber trees was the raw material first used for the manu facture of tyres, although synthetically produced versions tend to be used these days. In its natural state, latex rubber is extremely sticky, and only acquires the required degree of hardness, elasticity and wear resistance after it has been vulcanised (a process that ties

the rubber molecules together) and soot has been added. During several brief periods during the 20th century there were passing fads for white tyres, and tyres with a blue tread also appeared on the market for a short time. In both cases, however, the colour was only a layer that had been applied to the black rubber, and it tended to come off in parts.

Only very recently have manufacturers been able to replace the soot with a light-coloured silicate and to make tyres in any colour by the addition of a range of different pigments. However, tyres in novel colours are still quite rare because the silicate process is very expensive. Furthermore, market research indicates that most consumers prefer black tyres because they consider them to be more reliable.

When does fish smell fishy ?

The experts tell us that fresh fish doesn't smell at all. A smell only develops when fish has been in transit for a long time or after long periods in the freezer. The origin of the familiar smell lies in micro organisms that settle on the fish - specialised bacteria that transform some amino acids into amines by releasing carbon dioxide. This causes the simplest and most prevalent amino acid in fish - glycin - to turn into the simplest organic amine, methylamine. Unfortunately, methylamine has a strong smell - of fish.

The longer a fish is exposed to air the more active the bacteria are, and the stronger the odour. However, there is a simple remedy which most cooks add to a plate of fish a slice of lemon. The fruit acid in lemon provides a chemical counterpart to the methyl amine alkali, reacting with it to become amino salt. This captures the smell, because only free amine is volatile and able to give rise to the objectionable smell.

Why shouldn't you put metal objects in a microwave oven ?

The interior of a microwave oven is a closed metal space which completely reflects the electromagnetic microwaves, without heating up itself. Furthermore, the size of the interior of an oven is designed so that the microwaves from the source - and those reflected by the walls - add to and intensify the total microwave field. Metal items in the cooking chamber would screen off the rays and prevent the waves reaching the food that is to be cooked. Moreover, the waves would also cause the electrons in the metal to move, which could cause electrical discharges in the form of sparks. Thin foil and gold-rimmed plates especially should not be placed in a microwave oven, because their thin metal layer heats up particularly well and may even evaporate entirely.

What is a machine ?

Whether it's a block and tackle, a pocket watch or a gear system, all mechanical machines are more or less complicated combinations of six basic elements, or simple machines. These are the lever, the wheel and axle, the screw, the inclined plane, the wedge and the pulley. They change the size and direction of a mechanical force and transform one type of energy into another. Simple machines are known to have existed as far back as Ancient Greece.

What use is satellite technology to farmers ?

In the not too distant past, the only way a satellite could help a farmer was by providing a weather forecast. In more recent times, however, innovations have meant that space technology can offer farmers a lot more help especially if they are looking after large expanses of land A method has been developed in Australia, for example, where the quality of grazing land for cattle can be checked with the help of satellite data. Satellite sensors report on the state of the biomass in predetermined areas, which means that dairy farmers are able to react promptly by providing additional fertiliser when the need arises. Other researchers are working on 'guiding' farm animals from space. Thanks to special collars, cattle can be located and prevented - by a gentle electric shock - from crossing virtual boundaries. At the same time, the collars can document contact with other cattle or visits to the milking machine. Farmers who are prepared to spend a lot of time in front of a computer can even control a tractor from there, using a GPS system to keep the vehicle on course as it ploughs the soil fully automatically-or at least until the fuel runs out.

Which is the most economical form of transport ?

The most economical form of transport is one that goes furthest using the least energy. It may be a toboggan which slides down a slope unaided - not counting the climb back to the top - a glider that can stay aloft on favourable updraughts for hours once airborne or a solar-powered car that can cross an entire continent (Australia) using only energy obtained from sunlight. As a rough guide, the lighter the better seems to be the key.