Tom Robinson, age 15, of Charlotte, North Carolina, for his question:

What is meant by pressure?

Basically, pressure is a crushing force exerted on solids, liquids or gases. It can be measured in units of weight per square inch. Standard air pressure at sea level is 14 1/2 pounds per square inch. This is the weight of a square inch column of air from the ground to the top of the atmosphere. In the realm of fluids, air pressing down on liquid mercury makes it possible to construct a barometer, a standard instrument for measuring atmospheric pressure. The earth’s interior is under enormous pressure from the layers above it. At the outer core, this weighty pressure is about 100,000 tons per square inch.

In solids and liquids, pressure causes side effects, such as heat and compressing. In the realm of gases, these side effects are much more complex. The precise laws that govern the behavior of gases under pressure reveal a great deal about the nature of atoms. All these aspects of pressure involve particles of matter. However, we have borrowed this weighty word to express a human condition. In this realm, pressure means the exertion of outside influences or circumstances to push us toward this or that course of action.