Throughout history the worlds greatest thinkers have pondered the big questions in life: why are we here; is there a God; how can we know something; why does toast always land butter-side down? But philosophy and the thirst for knowledge arent just concerned with the metaphysical questions of life. They also seek to answer the mysteries of the physical worldfrom the sky above our heads to the idiosyncrasies of our own bodies. Here are ten little mysteries that have puzzled the worlds thinkers at one time or another. Science has explained most of them, but not all

Why is the sky blue?
Good question. The sky is blue because of a process called Rayleigh scattering. When light from the Sun reaches the atmosphere, it strikes the molecules in the air and is scattered in all directions. The extent to which light is scattered depends on its frequency. Blue light, which has a high frequency, is scattered many times more than other colours. As more blue light is scattered, it becomes the dominant colour, causing bright blue skies on clear days.

Why are rain clouds grey?
Its usually pretty easy to tell when its about to rain because the clouds in the sky turn an ominous grey colour. Clouds that are full of water or ice particles absorb light so that they appear grey from below. Those that contain relatively small amounts of water reflect more light, so that they appear bright white from the ground.

Why is a new moon invisible?
Just as half the Earth is always in daylight and the other half is in darkness, so around 50 per cent of the Moons surface is always illuminated by the Sun. The degree to which the sunlit portion of the Moon is visible from Earth depends on the Moons phases, which change as it orbits the Earth. In the phase called the new moon, the area of the Moon that faces the Earth is completely in shadow, so it cannot be seen.

What causes tides?
Have you ever wondered what causes the rise and fall of the tide? When the Moon is directly over a given point on the surface of the Earth, its gravity exerts a powerful pull on the water of the oceans below, causing them to rise above their normal level. Water covering the parts of the Earth furthest from the Moon is also subject to this pull, and forms another axis of high water. At both crests, the condition known as high water prevails, while along the circumference of the Earth perpendicular to the direct-opposite tidal axis, phases of low water occur.

Why do c**kerels crow at dawn?
Most birds like to take part in what is known as the "dawn chorus". For around half an hour after sunrise, birds make more noise than at any other time of day, although all birds, including c**kerels, continue to crow throughout the day. The reason for the chorus is largely territorial, and is stimulated not just by light, but also by a biological response (even c**kerels that are shut up in coops without light will crow at dawn).

Why do aeroplanes suddenly plunge during thunderstorms?
During normal flight conditions, the lift force produced by an aeroplane is equal to its weight, so the plane flies at a steady level. Most of the lift is produced by the wings, which are set at a slight angle to the oncoming wind. If the angle increases, the plane produces more lift and will climb, and if the angle decreases, the plane will descend. When a plane flies into stormy conditions, it will often encounter patches of rapidly rising or descending air. As the plane flies into these patches, the angle at which the wings meet the wind changes, causing the plane to suddenly climb or descend.

Why do leaves turn brown in autumn?
Throughout the summer months, the leaves of a deciduous tree are like miniature chemical factories. Using a process called photosynthesis, they capture energy in the form of light and convert it into chemical energy. This process is made possible by the presence of chlorophyll, the substance that gives leaves their green colour. As autumn approaches, shortening daylight hours and falling temperatures induce trees to prepare for winter. They grow a corky membrane between the branch and the leaf stem, which interferes with the flow of nutrients into the leaf. The production of chlorophyll declines, and this causes the green colour in the leaf to fade to a yellow. In some trees, such as oaks, the flow of sugar from the leaf is also inhibited; this increased concentration causes a reaction that produces chemicals called anthocyanins, a pigment which causes yellowing leaves to turn red or brown.

Why do we yawn?
The traditional explanation for this phenomenon is that its your bodys way of obtaining more oxygen when you are tired or when there is a lack of fresh air. The problem is that we dont only yawn under those circumstances. Top athletes may yawn before a big race and were just as likely to yawn when theres a plentiful supply of fresh air as when we are sitting on a crowded bus. Perhaps the answer to the next question can help