Swim caps serve multiple purposes. First, they protect your hair from exposure to chlorine and other pool chemicals. Second, they keep the hair out of your eyes and face while swimming, out of other swimmers' way and away from potentially dangerous tangles in the pool filter. Finally, for the elite swimmers, swim caps can reduce hydrodynamic drag enough to shave fractions of a second off their time. While this isn't a concern for recreational swimmers, it may be just the edge a competitive racer needs to help her toward victory.
Work one hand, and then the other, into the swim cap and lace your fingers loosely together. Because swim caps are slightly oblong in shape, the long axis of the cap should be oriented up and down once you've got your fingers inside the cap. Hold the cap in front of you.
Pull your hands apart to stretch the cap so it will fit over your head.
Bend down, bringing your head down to the cap. Place the front of the cap against your forehead and slowly pull the cap on over your head from front to back.
Stop when the cap is halfway on if you have long hair. Take a moment to twist your hair, which should be loose behind you, into a bundle and bring it up onto the back of your head with one hand, then continue on to Step 5 with your free hand.
Bring the cap all the way down over the back of your head. It should cover your hair completely to ensure a good seal. If you have long hair, there is no exact science to achieving this seal; just try to arrange the bundled hair on the back of your head so that it sits as evenly as possible and doesn't protrude beneath the edge of the cap.