4 Kings (or 4 other cards) are pulled face up from the top to the bottom of their stack.
When you do it again, one has mysteriously turned face down, with its red back showing. This is repeated and the card turns face up again, while a different one has turned face down. This continues until each one has turned face down, one at a time, and all are once again face up.
The question is asked, "Now what color are the backs?" They answer, "red". You turn the cards over and say, "No, this is done with mirrors!"... all of the backs have changed to mirrors!