According to folklore, breaking a mirror is a surefire way to doom yourself to seven years of bad luck. The superstition seems to arise from the belief that mirrors don’t just reflect your image; they hold bits of your soul. That belief led people in the old days of the American South to cover mirrors in a house when someone died, lest their soul be trapped inside.
Like the number three, the number seven is often associated with luck. Seven years is a long time to be unlucky, which may be why people have come up with counter-measures to free themselves after breaking a mirror. These include touching a piece of the broken mirror to a tombstone or grinding the mirror shards into powder.
