There is no more famous conundrum in special relativity than the Twin Paradox.  One twin travels at great distance at the speed of light and returns, much younger than the other twin.  Yet who is moving and who isn’t?  It is commonly claimed that acceleration is crucial to explaining this paradox, yet it turns out to not be the important point.  In this video, Fermilab’s Dr. Don Lincoln explains the real answer to this perplexing puzzle.

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CORRECTION: There is a typo in the video. At 10:23, it says that the x position for observer C at event II is γL, but it should be 2γL. Dr. Lincoln is suitably embarrassed and apologetic.