HOw do they do this trick

Question

How do they figure out your number?

http://www.milaadesign.com/wizardy.html

Answer 1

omg that is soo cool

Answer 2

You can only get a certain set of numbers by following their directions, so they mark all the possibilities with the same symbol (it changes eacch time you do it).  As you are so busy looking for your symbol and then staring at it, you aren't supposed to notice several other numbers with that same symbol.  These are all the possibilities, and they should be the same every time.