Minecraft wiki entry for Hitboxes, associated text: The mechanics of suffocation is best explained using hitboxes. If a solid block enters the "head" of the mob, then it begins to suffocate. The location of the "head" is marked with an extremely thin red box, so that is where a mob can be suffocated. If the mob does not have a separate outline for its head, such as a ghast, then it is assumed that the entire mob is a head. Therefore, a player can easily suffocate a ghast by pushing a block anywhere into its hitbox.