Judy Keith, age 11, of Jackson, Miss., for her question:

What holds the continents in place?

We are used to the idea that a block of wood will float in water. This is because of buoyancy. A block of wood is lighter, less dense, than an equal amount of water. A balloon rises because the gas inside it is lighter than the surrounding air. It floats in the air. The same laws of buoyancy apply to copper and mercury. A bar of copper floats in a dish of liquid mercury because it is made of lighter material.

In the same way, the continents of the earth float on heavier rocks below them. They are made of crustal blocks, the lightest of the rocks and minerals which make up our solid earth. They are supported b y layers of heavier rocks   hence they cannot sink.