Minecraft wiki entry for Mob_grinder, associated text: This grinder involves surrounding a 4x4 pit with pistons, one block above the floor of the pit. Have mobs pushed into the pit, and attach the pistons to a loop or a lever so that the pistons quickly go on and off, causing the entities inside the pit to fall into the floor and suffocate. This requires that there is a one-block tall space beneath the floor of the pit, so that the mobs can fall. This kills extremely quickly.