Minecraft wiki entry for Mob_grinder, associated text: The most important aspect of a fall damage grinder is the drop height. Different mobs need to be dropped different distances in order to kill them. Most common hostile mobs can be killed with a 24-block drop, although witches require 30 blocks. Some mobs such as zombies and skeletons can occasionally spawn with Feather Falling boots or armor that can change the required distance. Because of this, it is often better to make the drop further than necessary.