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Goofball Answer:

Hannah,

Actually sweetie, parachutes don't float. They fly. When the parachute opens and inflates with air, it takes on the shape of an airplane wing and actually flies through the air much like a glider or an airplane with a bad engine problem. Take a look at your sister's question about how airplanes fly to get a little more information.

There's actually a joke about how skydivers know when to "pull the string", as you put it. The saying is, "When the people look like ants, pull. If the ants look like people, don't bother, it's too late." But the real way we know how to deploy our parachutes is by using a device called an altimeter. The altimeter looks like a really big watch (see picture at right), but it measures air pressure instead of time. Before I get in the airplane, I set my altimeter to 0, which represents the ground. Once the airplane starts climbing, it takes us into higher and higher air, which is at lower pressure. As this happens, the altimeter's dial begins turning, marking the height above the ground. Once I jump, I begin falling lower and lower, into air at higher pressures, and the altimeter's dial begins turning back toward 0. But obviously I don't want to hit the ground, so I keep an eye on my altimeter and I open my parachute when the dial marks approximately 3,000 feet. At 3,000 feet I am still about 15 or 16 seconds away from hitting the ground. By opening my parachute at this altitude, I leave myself a pretty generous window of time to solve any problems that may occur with my parachute. And that's how we know when to pull the string.

Cheers,

topher