Minecraft wiki entry for Obsidian_farming, associated text: One of the first designs used the wither to break portions of the 5×5 obsidian spawn platform in the End, and regenerating it with a sand generator that pushed sand through the end portal situated in the Overworld. This method was patched however when the wither was made to shoot blue skulls that were capable of breaking any block. As such, this method works only in Minecraft 12w36a and several earlier versions. The following video is an explanation of how this method works.