A Rattleback is a half-ellipsoid object carved so that it will spin in only one direction. Accidentally discovered by archaeologists, this curious object was first dubbed a celt stone, named after the prehistoric axes and adzes they were studying. Idly spinning one of these ancient tools on the table, these scientists must have been surprised to discover that the seemingly normal piece of carved stone would spin freely in one direction but would reverse itself if spun in the other direction.