Minecraft wiki entry for Traps, associated text: This works because when a gravity-affected block falls on a fence post, the block stays as an entity. As a result, a player walking between the fence posts will fall through the sand entities, causing them to plummet. Diagonal hallways make it more likely to fall through the sand without landing safely on a fence post. Keep in mind that the sand is going to look half a block higher than a normal block, but this can easily be covered up. The best part: it does not need to be reset!