Minecraft wiki entry for Mob_grinder, associated text: In the figure on the right, a sign (or a button) is used to stop the water flow. The water pushes the mob onto the campfire. A hopper placed directly underneath a campfire pulls in any items dropped. The hoppers feed into a collection chest. The glass block traps mobs in the campfire space, and glass is useful for viewing the mob being killed. The half-block gap between the glass and the campfire allows for safe manual killing if needed (which gains experience for the player).