I love magic tricks.

A magician presents you with five cards. Each card has a red front side and a yellow back side, and each side has a number on it. The magician turns away and you select the five cards in any order and place them upright in a stand facing you, but a bar hides the number. You tell the magician you’ve selected your cards. The magician turns around, looks at the cards and instantly predicts the sum of the hidden numbers. The cards are revealed and the magician’s prediction is correct.

For example, suppose the magician sees the card configuration of (red, yellow, yellow, red, red) on the right:

He will correctly predict that the sum of the cards is 30.

The sum of the cards will always be 40 – (5 * number of yellow cards). So for three red cards and two yellow cards, the sum will be 40 – (5 * 2) = 30. The trick depends on the configuration of the numbers on the cards so that the difference between the red front and the yellow back is always 5:

Very cute. I saw this trick demonstrated on a YouTube video. The trick is surprisingly effective because you can’t easily see the card configuration. And the red-yellow color combination adds a kind of psychological camouflage.