Minecraft wiki entry for Mob_grinder, associated text: You may want to collect items automatically after the mobs have been killed. Any easty way to do this is to place a hopper at the end of your water flow or wherever your items drop out. A double chest can then be placed under or next to the hopper to contain the items. If there is a large area where the items can land, you may need multiple hoppers, or manage water flows to move items into a single destination hopper.