This program is both a puzzle and a magic trick. Once you know the solution to the puzzle, you can use this information to perform the magic trick.
When performed, the magic trick will appear to your audience as follows:
An audience member is asked to choose any five cards from a deck of playing cards. The magician takes one of the five cards. The spectator enters the names of the other four cards into the program and presses the "Guess Card" button. The result is the name of the fifth card.
How is this possible? Figuring this out is the challenge that makes this puzzle so interesting.
Please note that the secret to performing this trick is not provided.
Therefore, if you are not familiar with the secret, you should first treat this program as a puzzle and try to figure out how it works. Enter different cards to see if you can figure out the rule that produces the name of the fifth card.
For more information, visit its page in the iTunes App Store.