Minecraft wiki entry for Shelters, associated text: Using a fairly simple redstone circuit connected to a lever at one end, and one or more pistons at the other end, you'll be able to build some simple floodgates. If you extend the pistons first, and then place water, or even lava behind it, you can get rid of or even kill any mob passing by pulling the lever and letting the water or lava flow! For a quick morning mob clean-up and a strong sense of catharsis, you can place lava floodgates surrounding your entire shelter, facing out. One throw of the lever reduces the surrounding landscape to a barren wasteland! Not to be used in wooded areas.