Unbelievable Stargate Card Trick

This trick was invented many years ago by the late British magician Terri Rogers. She was also a ventriloquist, and you can find some good clips of her and her ventriloquist partner Shorty Harrison online.

This trick involves two playing cards that have been stuck together face to face. However a square hole has been cut through the middle.

By folding one of the cards, it can pass through the hole in the middle, although the cards are still (partly) joined together. When you open this card up again, the two cards have reversed their orientation, and are now joined back to back, i.e. you can now see the faces of the cards.

All done with topology. And a little bit of magic as well of course!