My Way Out Of This World by Larry Hass

Here’s what happens:

A deck of cards is fairly shuffled by the audience. Then the four aces are taken out and laid on the table to be indicator cards. First, the magician removes cards at random and the audience tells them to place the removed cards in the red pile or the black pile. For the second phase, no one looks at the cards as the audience randomly places those cards in the red or black pile. The balance of the deck is set aside, and when the piles are turned over all the cards are astonishingly found to be sorted into the correct pile.