David Cheatham, age 13, of Nogales, Ariz., for his question:

HOW LONG DOES IT TAKE TO LEARN SKYDIVING?

Skydiving is the sport of parachuting that is an outgrowth of military and stunt parachuting. In most cases, a first jump can be made safely after three hours of professional skydiving instruction.

The development of new jumping techniques that increase the safety of the jump and the stability of the parachutist in the air and of steerable parachutes of improved design have contributed greatly to the current popularity of the sport of skydiving.

A modern skydiving parachute is made with slits and holes in the canopy and deeply scalloped edges so that most of the shock of opening the parachute is eliminated. This design produces a slightly oblique instead of a vertical descent. By manipulating the shroud lines that attach the parachute harness to the canopy, the parachutist can alter both forward speed and rate of descent. Skydiving emerged as an international sport in 1951, when competitors from five countries met in Yugoslavia in the first world championships.