The Free On-line Dictionary of Computing (30 December 2018):

Banach-Tarski paradox

    It is possible to cut a solid ball into finitely
   many pieces (actually about half a dozen), and then put the
   pieces together again to get two solid balls, each the same
   size as the original.

   This paradox is a consequence of the Axiom of Choice.

   (1995-03-29)