XKCD: Pumpkin Carving

The Axiom of Choice, which is referenced in this comic, implies the Banach-Tarski Theorem (paradox), which states that it is possible to cut a sphere in a finite number of pieces, and reassemble the pieces into two spheres, each one of volume equal to the first one.  It is called a paradox because it is counter-intuitive.

Click http://xkcd.com/804/ link to open resource.