This activity requires a Magician (who knows how to do it) and a Player.[br]1. Somebody click "Twiddle Tiles" to generate a random 5x5 field of squares.[br]2. Magician clicks on remaining 11 squares to set their colors. Now the field is 6x6.[br]3. Magician looks away. Player clicks [i]any one square[/i] to change its color.[br]4. Magician looks at field and finds the square that Player flipped.

This is (somewhat ironically) a digital version of Activity 4: Card Flip Magic from the [url=http://csunplugged.org/]CS Unplugged[/url]. Go there for full instructions.[br][br]I made this applet because I don't have magnets to make a presentation set. If you're working on a flat surface (hello, document camera!), any two-sided object (playing cards, two-color-tiles, etc.) can do the same job.