Dolores Martin, age 10, of Dillwyn, Virginia, for her question:

Is it true that the continents are floating?

Corks float on water and a chunk of copper will float near the surface of liquid mercury. This is because water and mercury are heavier than these floating substances. The earth is made of solid substances and the heaviest ones tend to sink down to the center. This may explain why the rocks in the surface crust are the lightest ones. Long ago, scientists suspected that the vast continents of dry land were masses of light rocks floating on the top of heavier layers below.

They still think that this is mainly true. But in the last few years, earth scientists have discovered some amazing new things about the continents and the sea beds. The floors of the oceans seem to be stretching and changing their shapes. They also seem to be broken into large slabs. More investigation is needed to learn the whole truth. Both the continents and sea beds most likely float on heavier layers. But scientists suspect that the entire earth's crust may be broken into huge, slowly shifting slabs.