Christine White, age 9, of Niagara Falls, Ontario, for her question:

How many hairs are there on a human head?

Counting the hairs on a human head seems like an impossible job. For one thing, some people have very thick hair and a bald person has almost none at all. The best we can do is to count the hairs growing on a small patch of the head    and use this number to figure out the rest.

If you have a full head of healthy hair, there are about one thousand of them growing on each square inch of your scalp. This means that there are about 120,000 hairs on a full head of hair. Some human heads have fewer than 100,000 hairs and some have more than 200,000    and of course some human heads are quite hairless. Nobody can be certain about the exact number because each hair lasts only a year or so. Every day, a number of old hairs fall out and a number of new ones start to grow.