At least 4 people, but more people make for more fun
One player, the Leader, begins moving around with actions that the rest of the players must mimic. Anything — including wildly flailing his hands or furiously scratching his head — what the leader does, the others must follow. Those players who disobey, or lag behind the leader’s motions are out of the game. The last person standing becomes the new Leader.
Observe closely, and use the power of mimicry to advance to the winner’s circle.
Make it more fun
Set the game to music. Instead of outlandish bodily movements, have the leader demonstrate any of a number of dance moves that the followers must imitate.