Bloody Mary was some chick who dies I guess (and then I suppose they named a drink after her
?) and somehow it involves a mirror. Anyway, you turn off all the lights and stand in front of a mirror with only a candle and say "Bloody mary" six times and the ghost of her bloody corpse appears in the mirror.
Usually though the light plays havoc on your eyes and your own face ends up looking screwed and you run out of the house screaming like a 12 year old girl.
Bored kids with nothing better to do.