Minecraft wiki entry for Piston_uses, associated text: In a tunnel, mine out the ceiling an additional two blocks high. At the top of every part of the tunnel, place a sticky piston, then replace the ceiling with the same material used in the tunnel. This trap can be triggered in a number of ways, the simplest being a pressure plate or lever. If you choose to use a lever, this trap can also act as a door, because aggressive non-baby cannot enter. For this example, we use a pressure plate.