Why are some people ‘doublejointed’?-> The actual number of joints in your limbs doesn’t vary except in cases of very severe developmental abnormality. But the ligaments that hold together the bones on either side of a joint do vary in elasticity from one person to another. The young, and those that regularly stretch these joints through gymnastics or yoga, can achieve a range of movement that appears unnatural to the rest of us.Truly freakish joint positioning, known as hypermobility is either due to misaligned or malformed bones at the joints or Ehlers-Danlos Syndrome which is an inherited condition that affects the structure of the collagen protein in the skin, ligaments and bones. There are also some people that have reduced joint proprioception, so the brain misjudges how stretched a joint is and does not signal the muscles appropriately.
Why does cake and bread go hard, but biscuits go soft when stale?-> Whole books have been written on this. The basic answer for bread concerns the crystals of starch in the flour, which are gelatinised during baking (they take up water and become soft). This starch gradually recrystallises over a few days in a process called retrogradation, so that the bread becomes harder. The starch in biscuits also undergoes this process, but it can be overwhelmed by the effect of the sugar that is present in many biscuit recipes. Sugar takes up water from the atmosphere, and this is what causes biscuits that are left out to go soft. Cake can go either way, depending on the recipe, and both processes can be hugely affected by other ingredients in the recipe.
Can power be harvested from lightning?-> Yes, although it’s probably impractical because of such problems as designing a capacitor to store energy from a fleeting burst of power.
Would helium balloons float upwards on a spaceship?-> No, there is no ‘up’ on a spaceship, because there is no external gravitational field to tell us which way is ‘down’. In the absence of gravity there is no force to push or pull the balloon.
Would a metal plate in my head make it stick to magnets?-> No, because metals used for prosthetic purposes such as titanium are nonferromagnetic alloys. So you can relax, and feel free to have a metal plate fitted to your head with no fear of being stuck to a magnet.
Is there mathematics in music?-> Yes, there’s a host of connections between maths and music – from the relationship between the lengths of plucked strings and the notes they produce to the symmetry of Bach’s cantatas.
Can computers generate truly random numbers?-> Yes, and it’s because heat in electrical components causes electrons inside to move around unpredictably. This behaviour can be captured electronically and digitised as a sequence of truly random numbers.
Why can’t chickens fly?-> Wild chickens certainly can fly – and do. However, they have been selectively bredfor size for thousands of years and are now too heavy for more than a short flap to the top of a tree. Free-range chickens also often have the flight feathers on one wing clipped. This makes them fly in a short circle, which discourages them from flying away.
Why do pigeons bob their heads?-> The most likely theory is for the same reason that we move our eyes around – to stabilise the image of their surroundings while in motion. When a pigeon is walking on a treadmill, so that its environment remains relatively the same, its head does not bob. Not all birds bob their heads, though, so the issue is not yet fully resolved.
What exactly is an itch?-> Itching, or pruritis, is caused by any light skin stimulation that is just a few microns in size. It probably evolved as a warning mechanism to prevent insect bites. An itch provokes immediate scratching because it’s the quickest way to kill a mosquito trying to give you malaria.
Can déjà vu be explained?-> Haven’t we answered this question before? German workers in the 19th century suggested that déjà vu is a sort of cognitive burp. This occurs when the processes of sensation and perception, that normally occur simultaneously, somehow get out of sync. The modern take on this is that the ‘retrieval’ and ‘familiarity’ processes in the brain are not synchronised. But there are many othertheories, and no-one really knows, so at the moment the answer is no, it cannot.
Why do planes dim their cabin lights when they take off?-> The reason is to acclimatise passengers’ eyes to the dark. This isn’t just for comfort, but is a valuable safety procedure. If the plane had to be evacuated in an emergency, passengers’ eyes would be better suited to the darkness outside.
Why do we have five fingers and five toes?-> It’s probably nothing more than an accident. All tetrapods (the group of vertebrates that includes the mammals, birds, reptiles and amphibians) are descended from a pentadactyl ancestor. Pentadactyl means ‘five-fingered’ and this basic pattern has been preserved in all descendant animal groups. The common ancestor of the tetrapods was a lobe-finned fish living in the Devonian period, around 365 million years ago. The fossil record also shows fish with six and seven ‘fingers’ in their fins at around the same time, but there is no clear reason why the five-fingered form survived to become the template for land life. Possibly having fewer finger bones allowed each to become stronger and this helped when crawling out of the water. However, there is nothing magic about the number five and subsequent evolutionary pressures have driven many species to fuse fingers and toes together to form thicker, stronger hooves and claws that are better suited to their particular environment.
Why is my beer brown, but the head white?-> The brown colour comes from malt, which is produced by allowing barley grains to germinate and then roasting them. A low roasting temperature makes a light beer. A higher temperature makes a darker beer. Just be thankful that you aren’t living in the 19th century, when some publicans tried to shortcut the process by adding concentrated sulfuric acid to their light-coloured beers, carbonising the sugars to produce an instant dark brown colour, and instant stomach problems in those who drank it. The bubbles in the head are surrounded by a film of liquid beer, but the film is so thin that it can’t absorb enough light to affect the colour of white light as it passes through. White light reflected from the surface of the bubbles also stays white, giving it its overall white effect.
Does cheese give you nightmares?-> Any heavy meal before bed can make you spend more time in REM sleep and therefore dream more. But there is no evidence to suggest that cheese is particularly effective at causing dreams, good or bad. LV
What is the coldest place in the Universe?-> The coldest place in the Universe is in the Boomerang Nebula, a cloud of dust and gases 5000 lightyears from Earth. It has a temperature of -272°C (-457.6°F) and is formed by the rapid expansion of gas and dust flowing away from its central ageing star.
Is talking to yourself really a sign of madness?-> No, the phenomenon known as ‘private speech’, in which people talk aloud to themselves, particularly when stressed or alone, is perfectly normal.
Can dock leaves really sooth nettle stings?-> No, the myth of dock leaves soothing nettle stings arose because of parents’ desires to find something close by with which to placate their stung child.
Why does clingfilm cling?-> Clingfilm is either made from PVC or low density polyethylene that’s treated to make it stretch. When you unroll the clingfilm, some of the electrons on the surface of one layer get pulled away onto the adjacent layer. This creates patches of positive and negative electrostatic charge. Because clingfilm is a good insulator, this charge persists for quite a while. When you wrap the clingfilm around itself or another insulator (like glass) the electrostatic charge induces an opposite charge in the other surface and the two stick together. If you try this on a conductor, like metal, it won’t stick because the charge is dispersed.
Did the Big Bang make a bang?-> As sound is made up of wave-like changes in density, it’s impossible to hear anything in a true vacuum. Even so, the early Universe wasn’t a vacuum, being filled instead with hydrogen and ionised gas. The resulting sound of the Big Bang would be far too low-frequency to hear directly, but using real cosmic data and boosting the frequency, cosmologist Professor John Cramer of the University of Washington has simulated the sound, which can be heard at http://bit.ly/im0N.
How do you create artificial gravity in spacecraft?-> Studies of astronauts on long missions have shown that prolonged exposure to weightlessness weakens muscles and bones, prompting scientists to seek ways of generating artificial gravity aboard spacecraft. Even before the first space flights, visionaries such as Werner Von Braun suggested making spacecraft spin to create a centrifugal effect that feels like gravity. But experiments in the 1960s and 1970s revealed that rotation rates greater than around 2rpm tended to make people feel nauseous. This was bad news; such a slow spin rate meant that to generate an effect that could mimic Earth-like gravity, a spacecraft would need a diameter of around 450m. Engineers are still trying to solve the problem, but have had limited success. A team at the Massachusetts Institute of Technology has experimented with a ‘gravity gym’ – essentially a man-sized spin-dryer that astronauts can climb into to experience a short period of artificial gravity. The rotation rate must- be higher than the spaceship – 23rpm – so nausea is still a problem. There are other ways, such as making the spacecraft permanently accelerate at 1G or building a vehicle so large that it naturally generates its own gravity. But as yet both methods are far beyond our technical abilities.
Read the full 101 Greatest Questions of All Time in the March issue of BBC Focus. Find out more at www.bbcfocusmagazine.com101 greatest questions of all time: 1-20 101 greatest questions of all time: 21-40 101 greatest questions of all time: 41-60 101 greatest questions of all time: 61-80 
This photo made available by forensic archaeologist and anthropologist Matteo Borrini of Florence University, Italy, shows the 16th-century remains of a woman with a brick stuck between her jaws, as depicted in the computer generated image at left.
