Eric Ottum, age 12, of Falmouth, Me., for his question:

Why does oil float?

Oil floats on the surface of water because it is a substance which is lighter than water. We say that its density is less than the density of water. A cup of oil weighs less than an equal cup of water. If the oil contained suspended particles of some Very dense material, it then would be heavier than water and would sink to the bottom.

We call the ability to float buoyancy. In general, lighter and less dense substances tend to have more buoyancy than heavier substances. The rule applies to gases, some of which are heavier than others. Heavy carbon dioxide tends to sink to the lower levels of the atmosphere. Hydrogen is light and its buoyancy takes it up through the heavier air.