Minecraft wiki entry for Traps, associated text: When a slime of any size wanders around, it may fall into the hole. Since slimes cannot jump in water, it will not be able to escape. You can come by later and kill all the slimes. If you are not interested in the drops, just leave them into lower slime population. This trap is best on superflat survival worlds where slimes spawn frequently. In that case, water can be obtained in villages.