David Smith, age 13, Of Graham, N.C.., for his question:

What causes things to explode?

Solids, liquids and gases are states of matter, and the change from one state to another depends upon temperature. The changes come at critical points of temperature and are often sudden and dramatic. The most dramatic are those that suddenly change a substance from its solid or liquid state into gas. This is what creates an explosion.

Gaseous water vapor takes up at least 600 times more space than liquid water or solid ice. In some substances the volume ratio between the gas and its solid or liquid State is much greater. When the change happens suddenly with perhaps a detonation, the substance needs more room in a great hurry. It explodes outward with sudden and terrific force, shoving everything out of its way.