Minecraft wiki entry for Cave_spider_farming, associated text: One way to get the cave spiders to go into the grinder is by pouring water on every block on one side of the spawning area. Then, dig down 1 for every block on the opposite side, place a water source block on each side of the ditch, and place signs over each block in the ditch. Now, all the water should eventually flow to one block. If you want to make sure that the cave spiders don't climb the walls, place water sources all along the top of each wall.