A rattleback is a fascinating toy that can spin in both directions but will only rotate in one direction when it is spun in the other direction. This phenomenon is not new, and there are plenty of synonyms that describe it. Some of these synonyms include "celt," "turtle stone," "wobblestone," "spinthariscope," and "anagyre." While these words may not be as common as rattleback, they all describe the same principle of a spinning toy that has an unusual behavior. Whether you call it a rattleback, celt, or turtle stone, one thing is for sure: it is an entertaining toy that has fascinated people for centuries.