Shannon Ayotte, age 12, of Cardston, Alberta, Canada, for her question:

How much water is in the oceans?

Naturally, nobody has measured it cup by cup, or even gallon by gallon. But scientists have made an estimate, an educated guess, on how much water is contained in the world oceans. The unit used for the tremendous measuring job is the cubic mile. This, of course, is the volume of a cube measuring one mile on each of its six sides. The approximate figure for the earth's total ocean water is 326 million cubic miles. This is 97.2 per cent of all the earth's water. The rest is in lakes and streams, subsurface ground water and frozen in glaciers.

Ocean water, of course, is rich in dissolved salty minerals. This too has been estimated. Each cubic mile of sea water contains about 166 million tons of dissolved chemicals. In the total world ocean, this material amounts to 8O quadrillion tons  give or take quite a few tons.