Minecraft wiki entry for Mob_farm, associated text: Active systems have to change configuration to actually kill mobs, usually in the form of redstone devices. The most known might be the "Piston Grinder", which applies suffocation damage by pushing an opaque block into the head of the mob. While possibly faster than passive systems in killing, active systems usually have a limited capacity, and higher amounts of mobs might clog or even jam the system. The "Minecart Grinder" uses Minecarts to carry the mobs into one block high space, suffocating them. This method can jam up too, but is fairly amusing. You could also use lava at the above the hoppers so you get the loot before the loot burns automatically.