Here is a card trick. Your task is to figure out how this is done. Two people (who know the trick) do the following:
Player 1 shuffles a deck of cards and takes 4 at random, without showing Player 2.
Player 1 puts one card aside (still not showing Player 2), and puts down the remaining three cards. If she chooses, Player 1 can put each card either face up or face down.
Player 2 then announces the suit and value of the card that was put aside.
To help you determine how this trick is done, the following simulation allows you to enter the three visible cards and will announce the value of the hidden card.
You can find out much more about this through the column and article by Colm Mulcahy.