A deck of cards is shown blank on both sides.
The magician talks about how cards are printed - not by printing presses, but by magic.

At the magicians command, a card appears - but it only has a face but no back!
Then the magician "prints" a back but no face!.

Finally the magician shows what a deck looks like that a magician has completely,
printed by making all the cards have both faces and backs!

But a magician can always control the printing on the cards and with the snap of a finger they all turn blank again!

Bicycle Poker deck