Minecraft wiki entry for Piston_uses, associated text: One of the piston's most important uses is its ability to stop fluids. A floodgate is a device that does just that. Fluids don't destroy pressure plates, which makes them ideal for triggering this trap. In a tunnel, have a pressure plate activate an RS-NOR latch that blocks the player into that tunnel, which also minimizes other damage caused by the fluid. From the same output, have the floodgates open, dropping lava on the target.