Why have humans evolved to have less body hair than apes?
-> The only hairs that humans have completely lost are the vibrissae, or sensory whiskers. The rest of our bodies are actually covered with very short, very fine hair called vellus. The difference between humans and apes lies in the relative proportions and distribution of vellus hair compared with our longer, darker hair (known as terminal hair). There have been many different attempts to explain the relative hairlessness of humans, but so far none have achieved general consensus. It has been suggested that our hair was lost during a semi-aquatic period of our prehistory, that it was lost to allow improved cooling from sweating on the hot African savannah and even that we didn’t lose our hair until Neanderthal man started wearing clothes about 200 million years ago. This is all speculative however as hair doesn’t fossilise well so we don’t even know for sure if hairlessness is unique to humans.

Why can’t seagulls fart?
-> I can find no published study or even an oblique reference to the myth that suggests they might not! Seagulls, and birds in general, have a single opening called the cloaca, which serves for removing both the waste products of the kidneys and the intestines. This has a muscular sphincter to hold it closed, much like the anus. The bird’s digestive tract, though shorter than ours, still contains bacteria and these bacteria will produce gas. When the gas pressure exceeds the elastic strength of the cloaca sphincter, the result must inevitably be a fart. Possibly the reason seagulls might have acquired a reputation for never farting is that when a seagull poops in mid-air the person below suffers the immediate consequences. But with a mid-air fart, who’s to know?

What exactly is 20:20 vision?
-> If you have ‘perfect’ visual acuity, you have 20/20 vision, as measured by the standard eye chart devised by a Dutch ophthalmologist called Dr Hermann Snellen in 1862. The charts, which are still in use, have a single large letter at the top and lines of progressively smaller letters below. The first figure refers to how far away (in feet) the person whose vision is being measured is sitting or standing from the chart. The second figure refers to how far away a person with good vision would have to be and still be able to read the same line of letters as the person being tested. If you had 20/30 vision, a person with perfect vision could read at 30 feet the same letters that you can just make out at 20 feet. These distances are now generally expressed in metres, so that ‘perfect’ 20:20 sight would now be written as 6:6.

Could a human genetic mutation produce a superhero power?
->No, genetic mutations just cause a gene that codes for one amino acid chain to code for another slightly different chain.

Do opposites really attract?
-> No, people are likely to choose partners who are similar when it comes to three of the big five personality traits – agreeableness, conscientiousness and openness, extraversion and neuroticism.

Do lions purr?
-> All members of the cat family purr, although the Pantherinae subfamily – which includes the lion – can only purr while exhaling.

Are some people born lucky?
-> Luck is random, or it isn’t really luck. What changes from one person to another is our perception of luck and good fortune. Professor Richard Wiseman, Focus contributor and author of The Luck Factor has concluded that some people are better than others in acting upon chance opportunities and adopting a relaxed attitude that is open to new experiences. The very fact that these people think of themselves as lucky, tends to mean that they interpret their life in an optimistic way that exaggerates the effect of good luck and downplays any bad luck.

How many cigarettes would I have to smoke to become addicted?
-> Nicotine addiction is assessed using the Fagerstrom Test. This is a short questionnaire that principally assesses how many cigarettes you smoke a day, and how soon after waking you smoke your first cigarette. It takes more than a single cigarette to become hooked, but addiction in young smokers is typically established within a year of first experimenting with tobacco. LV

Why do clouds float?
-> The updraughts of warm, moist air that form clouds as they cool also serve to keep them in the sky. The total amount of water in a typical cloud weighs as much as 200 bull elephants, but it doesn’t crash down to the ground because the water is broken up into tiny droplets and ice crystals. Even the largest dropletsonly have a radius of 0.1mm. A droplet of this size falling freely would experience so much air resistance that its maximum speed would be a mere 30cm/s, but in a cloud the downward speed is balanced by the upward speed of the rising air. Only when many such droplets coalesce can they become big enough to fall as rain The air speed necessary to keep clouds ‘floating’ depends on the type of cloud. Flat, spread-out stratiform clouds are formed and supported by weak air currents rising at only a few centimeters per second. Cumulus (or convective) clouds, which are the ones responsible for heavy rain and storms, contain bigger droplets and need updraughts of a few metres per second to support them.

At what point does space begin?
-> Amazingly, more than 50 years after astronauts started exploring space, there’s still no internationally recognized legal definition of where they have ventured. NASA traditionally awards anyone who reaches an altitude of 80km ‘astronaut’s wings’. During the 1960s eight pilots from NASA’s X-15 experimental rocket plane were awarded this accolade, like the astronauts of the Mercury, Gemini and Apollo programmes, with pilot Joe Walker twice reaching a height of more than 100km in 1963. Most experts agree that missions to this altitude constitute genuine spaceflight, and it may yet become the legal standard, with lawyers in Australia in 2002 becoming the first to adopt 100km as the definition of where space begins.

Why do our noses run when we eat hot food?
-> A runny nose is normally caused by streaming eyes draining through the tear ducts into the nose. The watering eye’s response is mediated by the trigeminal nerve, which is the main facial nerve and has branches in the mouth, nose and eyes. The response probably evolved as a way of flushing the eyes and nose of irritants. With hot food, that irritant is the capsaicin oil; in cold weather, the drying effect of the wind is to blame.

How slow could I waterski before I’d sink? (I weigh 65kg)
-> A back-of-the-envelope calculation based on Newton’s Laws of Motion gives a figure of 20km/h. The lift force that keeps you up is a reaction to the force that you have to exert to move water out of the way as you travel through it. Its value depends on your speed, the area of ski in contact with the water, and the angle of attack of the skis with the water. From water-skiing photos and films it looks as though a typical angle of attack is around 15°, which gives a speed of 20km/h to just keep your weight up. Don’t take this figure too seriously though, as it doesn’t allow for frictional drag, bow wave effects or ski shape. It seems to fit with experience, however, and also with the claim that an elite K-4 kayaking team could tow a light waterskier at a record speed of just over 20km/h.

Why do humans laugh when tickled?
-> Most species respond to tickling or other light touches by withdrawing to avoid the attack on the vulnerable area. It appears that we learn to laugh at tickling as children only when we perceive the tickling as a mock attack that is actually an act of personal closeness. Interestingly, recent research by the cognitive neuroscientist Sarah-Jayne Blakemore has shown that we can’t tickle ourselves, no matter how hard we try.

Do fish drink?
-> Yes and no. While freshwater fish absorb more than enough water by osmosis through their gills, saltwater fish do drink because they lose water by osmosis.

Are Inuit loos made of ice?
-> Inuit toilets are just the same as yours or mine. The closest most Arctic people get to an igloo is watching Nanook of the North on DVD.

Can giraffes swim?
-> Yes, the BBC’s Big Cat Diary crew has filmed a giraffe swimming a short distance in Kenya’s Mara river.

Does playing music to plants help them grow?
-> No, there are a few plants that will react to sound vibration, for example Mimosa pudica, but musical appreciation is a different thing entirely.

Does peeing on a jellyfish sting really ease the pain?
-> No, urinating will probably cause even more ‘nematocysts’, tiny spring-loaded harpoons that inject venom under your skin, to fire!

Why do all planets and moons spin?
-> The reason is tied to the origins of the Solar System as a primordial Sun surrounded by initially randomly swirling clouds of dust and gas. Pulled towards the Sun by gravity, these clouds became denser, with internal collisions leading to a preferred direction of motion. Like water spiralling round a plughole, the collapsing clouds swirled in this direction at an ever-faster rate, eventually becoming dense enough to collapse under their own gravity and form spinning planets and moons. The one exception is Saturn’s moon Hyperion, which seems to have undergone a very violent impact, turning it into a potato-shaped rock that tumbles chaotically through space.

How do we know the speed of light is the same for everyone?
-> This is the assertion behind the theory of special relativity – and it’s not obvious. The speed of a train may be 200km/h to a stationary observer, but 400km/h relative to a train coming the other way. Einstein’s claim that the speed of light is the same for everyone, regardless of how they’re moving, was confirmed by experiments at CER N in Geneva. Sub-atomic particles were accelerated to 99.975 per cent of the speed of light, but when the speed of light they emitted was measured, it turned out to be 300,000km/s – the same as when the particles are stationary, as Einstein predicted.

Read the full 101 Greatest Questions of All Time in the March issue of BBC Focus. Find out more at www.bbcfocusmagazine.com

101 greatest questions of all time: 1-20
101 greatest questions of all time: 21-40
101 greatest questions of all time: 41-60