Minecraft wiki entry for Mob_grinder, associated text: Using pistons, it is possible to push blocks into mobs so that they suffocate. In the shown example design, a piston pushes a mob underneath a sticky piston holding a block. The sticky piston extends at the same time to "smash" the mob and cause it to die from suffocation at 2 points per second. The water used to push the mobs can also be used to wash the items to a hopper for item collection. This setup works only for tall mobs, but the same principle can be used for smaller mobs as well.