Crystals called otoliths—"ear rocks"—normally attached to balance-sensing cells in the inner ear get dislodged.

Symptoms: Episodes that last a few seconds, occur only in certain positions, and don't affect hearing.

The fix: The Epley maneuver. Your doctor will cradle your head as you quickly lie back. She'll turn your head to one side, wait 30 seconds, then turn it further, which repositions the otoliths. It's effective 80 percent of the time with one treatment, nearly 100 percent of the time when repeated. (You can learn a home version.)