Nearsightedness is a common vision problem where you can see things clearly when they're close to you, but objects far away look blurry and out of focus. The medical term is myopia. If you're nearsighted, you might be able to read a book just fine, but struggle to see what's written on the classroom whiteboard from the back of the room. A nearsighted baseball player might hit well but have trouble tracking fly balls in the outfield.

This happens because of the shape of the eyeball. In nearsighted eyes, the eyeball is slightly longer than normal, or the cornea (the clear front part of the eye) is curved too steeply. This causes light entering the eye to focus in front of the retina instead of directly on it, making distant objects appear fuzzy.

Nearsightedness often develops during childhood and may worsen as kids grow, then usually stabilizes in the late teens or early twenties. It's very common: about one in three people is nearsighted. Glasses or contact lenses correct the problem by adjusting how light enters the eye, bringing distant objects back into sharp focus.