Marilyn Hurrell, age 14, of Neepowa, Man., Canada, for her question:

How do we measure height above sea level?

The surface of the sea is higher in some places than in others; what's more, it moves up and down with every tide. Sea level is the average level of the ocean based on countless readings. The land is measured in feet above sea level. It is possible to figure the height of a peak from the length of its shadow, but usually altitude is measured with an altimeter.

This gadget measures air pressure, which becomes less and less as we rise above the earth:. Standard air pressure at sea level is 14.6 pounds over each square inch. It decreases at a rate which can be figured on a scale. If the altimeter shows 10.35 pounds of air pressure, then the mountain peak We are measuring is three and a half miles above sea level.