Minecraft wiki entry for Shelters, associated text: An experience farm uses water currents to push most mobs into a central area. With mobs in the nether, pistons are used. Use a piston crusher or a long drop to get the mobs to a ½ heart. Mobs can be killed by hand or with a splash potion of harming (Healing for undead mobs) to get XP. Note: This is more efficient with multiple spawners, as more spawners mean more spawning opportunities. You also obtain drops from each mob killed. This can also be done with large dark rooms with platforms. The mobs spawn and fall into water currents, where you can choose what happens to them once they are out of the spawning chamber.