Why do crackers crack?

... and 20 other Christmas questions answered

Will champagne really stay fizzy if you put a spoon in the bottle? How do you make the perfect snowball? And why do we nod off after Christmas lunch? Mick O'Hare, author of the runaway best-seller 'Why Don't Penguins' Feet Freeze?', reveals the science behind some seasonal conundrums

Why do people always feel sleepy after eating Christmas lunch?

After a meal, the levels of glucose in the blood are obviously high, and high blood glucose levels can switch off the brain cells that normally keep us awake and alert. Orexin neurons in the brain produce proteins, called orexins, which play a pivotal role in wakefulness. Glucose inhibits orexin neurons, making us feel more sleepy.

Sleepiness is the body's way of telling us to relax and conserve energy once we have found and eaten our food, and probably developed as an evolutionary response to being sated after a meal. By the same token, when we are hungry, the neurons are not inhibited, and we are alert enough to go in search of food - a sensible evolutionary precaution.

When you cook a Christmas pudding in the traditional way, how come the water in the pan doesn't soak through the cloth?

Two key factors are at work here. In the pan, bubbles form as the water boils. Just as the bubbles in a glass of lemonade cling to the straw, so the bubbles in the pan form on the rough surface of the pudding. Spots where gas pockets can grow to form bubbles are called nucleation sites. These can be anything from small dust particles to tiny salt crystals or scratches on the surface of glass.

These bubbles become trapped between the pudding and the cloth in which it is wrapped, forming an impermeable layer of steam that drives any water back outside the cloth. Fat also migrates from the pudding into this space, adding to the waterproofing.

For these reasons it is important that if you are cooking your pudding in this traditional manner you immerse it into already boiling water. If you try to heat the water from cold with the pudding already in the pan, water will have quickly penetrated the cloth and made the pudding soggy by the time it comes to the boil. (By the way, machined glass is now so smooth that drinks glasses are manufactured with deliberately rough inside surfaces - otherwise, bubbles would not form on the inside of them, so they wouldn't look so good.)

Are candles environmentally friendly?

Ordinary candles are generally made from paraffin wax, a by-product of the oil industry. Consequently, burning the wax releases carbon dioxide from the long hydrocarbon chains from which it is created - this adds to global warming as well as releasing soot particles which can be damaging to health. More environmentally friendly candles can be created from soya oil or beeswax but the cost of transporting them to shops means that electricity - especially from a renewable source - is the environmentally friendly option for lighting.

How does a teaspoon inserted into an opened bottle of champagne ensure it keeps its fizz overnight?

It doesn't. This is an urban myth perpetuated by uncontrolled experimentation. People leave spoons in their bottles and find that the wine is still fizzy the next day, so they presume the teaspoon was the reason. However, if you leave an opened bottle of champagne overnight with no teaspoon in the neck it will be just as fizzy. The truth is that champagne takes a long time to lose its fizz. It's not uncommon to attach significance to apparently linked events, which is why science always uses controls to separate real data from presumed data. This experiment works just as well with cava, and as two bottles are needed, it's significantly cheaper that way.

Why does frost usually form only on the edges of holly leaves?

For water droplets to form in damp air, the air needs to come into contact with a cold object - in this case, the holly leaf. At the sharp tips and edges, the surface area is greater, exposing them to more of the damp air. The leaves are also waxy on their upper surfaces, which causes any droplets that form there to just run off. But in colder temperatures, at the tips, the water settles and freezes, forming to frost.

Why are we supposed to open red wine to allow it to breathe before drinking?

Once wine is exposed to the air, volatile and aroma-bearing substances start to evaporate, so that we can enjoy the bouquet. This process can be enhanced by decanting the wine. Breathing also allows red wine, if stored in a cool place, to come up to room temperate. Too much breathing, however, and the wine will become oxidised - and is on its way to becoming vinegar.

What's the best way to open a Brazil nut without destroying it?

The cross-section of a Brazil nut is a rough equilateral triangle, with a slightly shorter base. Inside the shell, the nut is attached at the base of this triangle, but there's more room along the apex. Most people try to crack the nut by squeezing the crackers across the base, which transfers the force directly to the nut, which is attached at this point - hence many destroyed nuts.

But if the crackers are applied across the longer sides, the shell will split neatly. If the whole shell doesn't come away, do the same across the other side. You'll have more intact nuts than broken ones. Incidentally, the Brazil nuts you will find inside chocolates and nut selections this Christmas have had their shells removed industrially. The nuts are boiled until the shell softens and then fed through rollers carrying the exact tolerance required to crack the shell.

Why do helium balloons go flat so quickly?

Helium gas is not only very light (lighter than air - hence balloons float if filled with helium), it is also monatomic. This means that, whereas an oxygen particle contains two atoms (hence O2), all helium's particles are made of a single atom, and that atom is very, very small, with a diameter of only 0.1 nanometre - that's one ten-billionth of a metre. This means that helium will diffuse through even the metal films used for making special party balloons. This is also why helium is used industrially to check for leaks in laboratory vacuum systems. You could use air - mainly oxygen and nitrogen atoms, which are much bigger - in your balloons but, of course, they wouldn't fly so well...

Why does sparkling wine or beer poured into a dry glass froth up at first, but not when you top up your glass?

Beer, sparkling wine and other fizzy drinks are liquids supersaturated with gas. This gas comes out of solution when the drink is shaken or if it encounters nucleation sites. Once the inside of the glass is wet, any salt crystals will have dissolved and any dust particles will have been washed away, so there are fewer nucleation sites, while any scratches that remain will have been coated with liquid so any fresh champagne can only reach them very slowly, by diffusion. Bubbles will still be produced, but at a rate that is too slow for a cascade effect to come into play and the drink will not froth over.

The shortest day of the year in the northern hemisphere is usually 21 December, but the earliest sunset is on 14 December. Why?

There are two properties of the Earth's motion around the Sun that give rise to this effect. The first is the eccentricity of the Earth's orbit, the second the tilt of the equator to the Earth's orbital plane. These have the combined effect of meaning that not all days are exactly 24 hours long (although over the course of a year the average remains 24 hours). In December the average time of a day is approximately 30 seconds short of 24 hours. Because this difference is greater than the daily change in sunset times, it actually means that the earliest sunset occurs a few days before the shortest day.

What causes the rivulets of alcohol, known as "legs", on the inside of a wine or spirit glass?

The answer to this question was discovered by James Thomson, brother of the more famous scientist Lord Kelvin (of temperature fame) in 1855. The effect is caused by surface tension. Alcoholic drinks are, in effect, mixtures of alcohol - in the form of ethanol - and water, from which the ethanol evaporates faster. The different rates of evaporation induce conflicting forces between the more watery regions of the drink and the more alcoholic regions. The legs are the result of the watery and alcoholic regions being pulled apart - the increasingly watery liquid starts to trickle back down the inside of the glass as the ethanol evaporates.

Thomson's discovery brought him little fame. This kind of surface tension convection is now known as Marangoni convection, named after the scientist who worked on similar studies 20 years later.

What causes the northern lights, and why are they multicoloured?

The auroras, as both the northern and southern lights are known, are created from electronically charged particles from the Sun being carried on the solar wind to Earth. On entering the atmosphere, they are attracted to the magnetic poles where they concentrate.

While on their journey the particles interact with the different atoms, such as oxygen and nitrogen, that are present in the atmosphere. As these atoms are struck by the particles, their electrons are knocked away, leaving them in a charged or excited state. These excited atoms emit radiation at different wavelengths which create the various colours that are seen in the sky.

The colour you will see in the sky depends on the type of atom being struck and the altitude at which the collision occurs. Blue is created by nitrogen being struck at lower than 100km, green/yellow when oxygen is struck between 90 and 150km high, and red when hydrogen is struck at above 120km. When many gases are present, you get a multicoloured show.

How does the skin form on custard?

The skin is merely dried custard. When custard is freshly made and still hot, water evaporates from its surface, but water throughout the rest of the mixture stays tightly bound to the carbohydrate in the cornflour and protein in the egg, forming the gel-like texture of custard.

At the surface, however, things are different. As water evaporates from this region, the custard is unable to draw up more water from below, as would normally happen in such convection systems, and it dries out. Because the gel below holds heat extremely well, more and more water evaporates from the surface. This is why a skin forms quickly on custard left uneaten. The good news is that the skin is, in effect, concentrated custard, so it tastes much better than the wetter stuff below.

Can water detoxify you after Christmas excess?

The minimum volume of urine required by the kidneys to excrete the waste products of the human metabolism is about half-a-litre a day. Yet, because we lose a further half-litre of fluid through breathing, sweating and defecating, we need to replace our total lost fluids by drinking about a litre a day. Drinking more than this merely dilutes the urine - the same amount of waste products (or "toxins" as they are popularly known) are still removed from our bodies.

The idea of human "detoxification" is a current fad; however, there is no evidence to support the argument that our kidneys cannot cope with the input and output of normal, or even excessive, consumption as long as the latter is not on a long-term basis. Drinking more water than needed makes no significant difference to the elimination of waste products from our bodies.

Sometimes it's easy to make snowballs, other times it's difficult. What causes the difference?

Snow needs to be just the right temperature for snowball creation - warm enough for it to melt slightly under the pressure of the thrower's hands, but cold enough for it to reset once the pressure is released and therefore maintain a snowball shape. In extremely cold conditions not enough pressure can be induced by the thrower and the snow remains a powdery, crumbly mess. In conditions that are too warm, the slushy snow will melt as it is squeezed into shape. Temperatures in a British winter, hovering at or around freezing point, are just about ideal for snowball formation (at least, they were before the onset of global warming).

Why is it colder at the South Pole than the North Pole?

The northern polar region consists mainly of frozen sea ice. There is no land at the North Pole, it rests at sea level at whatever depth of ice is present at any given time. The southern polar region, by contrast, consists of the landmass continent of Antarctica and, as such, has hills, mountains and plateaux.

Because temperature decreases with altitude, and the South Pole is at 2,800m above sea level on top of a large mountain range, temperatures at the South Pole are invariably colder than those at the sea-level North Pole. Weather conditions and atmospheric circulation patterns add to the difference. Average winter temperatures at the South Pole are minus 60C; those at the North Pole are about 30C warmer.

Why do crackers crack?

Crackers contain two strips of card that are pressed together at their tips. The tips are treated with combustible chemicals and coated in fine sandpaper. When the ends are pulled, the two pieces of card separate, and the friction from the sandpaper heats up the chemicals which cause the bang.

How do they make the transparent ice cubes you see on television advertisements for drinks?

They don't, they use glass or Perspex so they don't melt under the studio lights. However, if you want to try to make some at home it can be done. Ice made in your freezer will probably be cloudy because it has gases dissolved in it which are forced out of solution as the water freezes. This explains why most of the bubbles and particles are concentrated in the middle of your ice cubes - this is the last part to freeze.

In order to have a decent chance of creating clear cubes, use boiling water which will remove some of the dissolved gases. Then freeze this water very slowly so that there is no steep temperature gradient and the bubbles, when they appear, do not get trapped but have time to move through the liquid before being frozen into place. Commercial ice-cube machines create clear cubes by only freezing a small portion of water at a time then trickling more over its surface so that the gas-carrying component just runs away while layer after layer of non-gassy ice builds up.

So many people dislike Brussels sprouts - are they really good for us?

It's true - sprouts are healthy. Moreover, the worse the sprouts taste, the better they are for you. Sprouts contain sinigrin (a glucosinolate compound derived from glucose). During digestion, these are broken down into the volatile molecule allyl isothiocyanate, which can suppress the development of damaged, pre-cancerous cells. Allyl isothiocyanate is largely responsible for the smell and taste of sprouts, along with another glucosinolate called glucoraphanin, which breaks down into sulphoraphane, which neutralises carcinogens. This means the more you dislike your sprouts, the more good they are doing. (Broccoli has many similar properties without the bitter taste - now there's an idea for Christmas lunch...)

Why do snowflakes always form six-sided symmetrical shapes?

No one knows. Ice forms hexagonal crystals because of the shape of the molecular crystal lattice created when water freezes at normal atmospheric pressure (other shapes form at different pressures). Snowflakes are loose collections of many of these tiny crystals.

In an environment where heat is being withdrawn at a slow and constant rate, water molecules join a six-sided lattice and fill the gaps in the lattice in an orderly, geometric way. However, if the ice is forming very rapidly, such as in a freezer, a dense mass of interlocked crystals will form with no apparent lattice structure - look at the ice cubes in your gin and tonic, no snowflake shapes in there. Unfortunately for Christmas-card designers, very few snowflakes are fully symmetrical - varying pressure and temperature conditions ensure that one arm or corner will be longer or shorter than the others. The varying microclimatic conditions also explain why two snowflakes can never be the same.

Why do butchers hang game to improve its flavour?

When hung in cold storage, the meat becomes more tender as cellular enzymes in the muscle tissue break down. The animal must have been bled, gutted and skinned as quickly as possible after it has been killed though - otherwise digested food and putrefying microbes would form gas in the intestines which can permeate the lining and spoil the meat, as anyone who has smelled roadkill will recognise. Leaving the skin on the body for a long period can also cause the meat to retain body heat which leads to souring. Lastly, if not properly bled, any blood left in the carcass will coagulate and remain in the body tissue.

Because the inside of the meat is sterile, any bacterial rotting proceeds slowly from the outside inwards and this layer can be trimmed by the butcher. In some cases, meat can be overhung and can drop from the bones to form an unpleasant mess. Even so, some people relish the flavour of overhung meats such as the "high hare". This is a furred, but cleaned and eviscerated, hare that is hung for longer than usual. The fur, acting as a sterile wrap, limits inward decay.

Why Don't Penguins' Feet Freeze?, a collection of columns from the 'New Scientist' columns edited by Mick O'Hare, is published by Profile, 7.99. To order a copy at the special price of £6.99, including free P&P, call Independent Books Direct on 01326 569 444