Here is a card trick. Your task is to figure out how this is done. Two people (who know the trick) do the following: 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.
Enter the first card: of

Enter the second card: of

Enter the third card: of

You can find out much more about this through the column and article by Colm Mulcahy.