Debbie Weaver, Age 11, Of Fortville, Ind., for her question:

What is the cause of air pressure?

All solids, liquids and gases are made of particles of matter, and all particles are attracted to each other by the force of gravity. The matter in a boulder is packed densely together, and this density causes a strong attraction between the boulder and the massive earth. This is what gives weight to the boulder. The gaseous air also is made of matter, but the tiny particles are widely separated. The air is less massive, less heavy than a dense stone.
Nevertheless, the air has weight, and it is the weight of the earth's blanket of atmosphere that causes air pressure. This air pressure is the attraction of gravity between the earth and the particles of matter in its atmosphere. Its total weight is 5 quadrillion tons, and the average pressure of this weight above each square inch at the earth's surface is about 14.7 pounds.