An early kind of burglar alarm - trip stairs. The idea was that when a robber broke in, he would try to sneak quietly up the stairs in the dark, not knowing that on the way up there would be steps close together (like the second and third from the top), causing him to hopefully trip and wake someone up. It apparently worked pretty well.
An early kind of burglar alarm - trip stairs. The idea was that the unexpected change in the level of the stairs would foil the would-be robbers by causing them to trip in the darkness and wake someone.