Pamela Brimeyer, age 9, of Holy Cross, Iowa, for her question:

Why must a plane cabin be pressurized?

When a plane flies high, the cabin is sealed shut and extra air is pumped inside it. This is what we call pressurizing. If this were not done, the air up there would be too thin to breathe. The unhappy passengers would become dizzy from lack of oxygen    and some of them would pass out. This is because the air gets thinner as we climb higher above the ground. People who climb high mountains must carry along tanks of oxygen. The air gets uncomfortably thin when they reach about three miles above the level of the sea. When they climb another half mile or so, it is time to start using their extra oxygen.

Modern planes fly six or seven miles above the level of the sea. Up there, people need extra air to match the dense air they are used to breathing on the ground. Pressure is caused by pressing weight. And even the filmy air has weight. We call this air pressure because it presses onto us from all sides. In the cabin of a high¬flying plane, we need extra air for breathing