Minecraft wiki entry for Wither_cage, associated text: There are two types of skulls that the wither shoots: black skulls which come from the side heads, and blue skulls which come from the center head. Black skulls are able to destroy weak blocks like wood and dirt. Blue skulls are able to destroy stronger blocks like stone and iron blocks. In most designs that work with skulls, some of the heads are intentionally “distracted” by other mobs. Keep in mind that the wither can sometimes hit itself with its own black skulls, which could cause unwanted block breaking.