So this guy walks into the main offices of NASA. "I've got an idea," he says. "When astronauts have to step outside for a space walk, why not have them carry suction cups? They could inch their way along the outside of the capsule and never have to worry."

Well? What do you think?

It sounds like it might work. After all, some very adventurous people have used big suction cups to climb the fronts of glass buildings before. However, they'd better not climb too high, because the higher you go, the less effective a suction cup will be. In space they're no good at all!

Why?

The answer is in how a suction cup works in the first place. It works because of air pressure. Air pressure is the weight of the earth's atmosphere pressing on things. We don't feel the air pressure on us, but that's because it's evenly balanced inside and out. In fact, there is an atmospheric pressure of 15 pounds per square inch on everything around you, including you.

When you set the suction cup down on a surface, the air pressure is balanced on all sides. Force the air out of the bowl, however, and now the air pressure is all concentrated on one side. The weight of the earth's atmosphere is actually what holds the cup against a smooth surface!

That's neat in itself, but you may see the problem. Up on top of a high mountain there is less air pressing down, so suction cups won't work as well. Out in space there is no pressure at all. A suction cup wouldn't hold for an instant.