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Q How much heat does the human body give off?

Curiously, men typically give off more heat than women - between 158 and 167 kilojoules per square metre of skin in men and 150-158 kilojoules per square metre in women. Adult men have about two square metres of skin, and so give out about 326 kilojoules from their bodies each hour - that is, just over 90 watts.

[Editor's note: the previous version of this answer was too low by a factor of 1,000. Many thanks to those who pointed out the error.]

Q Why was 24 hours (1/15th of 360 degrees, four minutes equivalent to one degree of longitude) chosen to divide up the day, when 36 hours (1/10th of 360 degrees, 6 "minutes" = 1 degree longitude) would seem more logical? (asked by C Douglas Goode)

Historical questions are always difficult to answer definitively. But most Western civilisations - Greeks, Sumerians and Babylonians, Egyptians, Romans and western Christendom - assigned 24 hours to a day, 12 of daylight and 12 of darkness. (It didn't matter that the length of an hour varied according to the seasons.) It might have been more convenient in retrospect if time had matched the units of rotation, but they came from different sources. The figure 24 came from 2 times 12, and was derived in Babylonia from the Sumerian sexagesimal method of reckoning (based on gradations of 60 - 5 times 12 - not multiples of 10, because they had a base system of 6). Babylonians then subdivided each section of the day into 30 gesh. This was then doubled to give 60 minutes some time later - but we don't know exactly why.

Q How big is the Great Pyramid in Egypt?

The Pyramid of Cheops, also known as the Great Pyramid, is 147 metres high, with sides 230.4 metres long.

Q What is hair made of?

Hair is made from cells called epithelial cells, which are arranged in three layers. The innermost layer is the medulla, the middle layer is the cortex and the outer layer is the cuticle. The medulla is mainly soft keratin (a protein) and the cortex and cuticle are mainly hard keratin. This structure has great strength - a strand of hair is stronger than an equally thick strand of nylon or copper wire.

Q What determines how tall you grow?

Your adult height is determined by genetics and nutrition - with genetics playing the major part. If you are tall but your parents aren't, it's very likely that somewhere along your family line you had tall relatives, so the genes for a tall person are there even though your father and mother are of average height. But nutrition can also be a factor. Some researchers are testing whether there is a particular stage during development when a good diet can greatly affect your height. Nutrition becomes a limiting factor on height only when malnutrition occurs or there is a lack of essential vitamins and minerals; so it's only partly true that there is a relationship between how much you eat and your height.

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