Minecraft wiki entry for Slime_farming, associated text: The first step in constructing either farm is the spawning place. In most farm designs, spawning platforms are created which spawn slimes that are then moved to a killing trap, usually a drowning trap to split larger slimes down and dispatch the smallest size. Spawning pads are usually constructed with a 2½-block gap using slabs, to allow all types of slimes to spawn. It is important to illuminate the spawn platforms a bit to allow slimes to spawn while preventing other hostile mobs from spawning. It is also recommended to illuminate horizontal surfaces in a 128-block radius around the farm, to keep the hostile mob cap as low as possible.