A skydiver’s body is in a delicate balance. Gravity tugs her downward with a constant, relentless force, while air resistance pushes her upward. The faster she falls, the more air resistance there is, so eventually these two forces cancel out. Then she falls at a constant rate, her terminal velocity. To link up with other skydivers in mid-air formations, skydivers need to control their terminal velocity very carefully. After all, if you’re falling at a hundred and ten miles per hour, and your friend is falling at a hundred and twenty, there’s no way you can link at all. As it turns out, your terminal velocity is determined by exactly two things–your weight, and the amount of surface area you expose to the ground. While a skydiver can’t do anything about her weight mid-fall, she can control how much area she presents downward. Stomach down, with arms and legs spread out in a kind of belly-flop, the average skydiver is likely to fall at around a hundred and ten miles per hour.