The Simple Mob Trap.

There's really no explaining. Walk in, Get trapped, drown. Build it by: Find a flat spot, place IRON DOORS while walking backwards, clockwise, or walking forward, counterclockwise. The place a pressure plate on the bottom, and then place two bedrock (In creative) or obsidian on top of the plate. Mine the block right above the plate, not the top one. Then place water right above the plate. Then if you walk in in survival, you will die. By drowning. You can kill mobs (HOPEFULLY) with this or you can kill noobs in a PvP multiplayer server.

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