Math Based Magic Trick

This one is a favorite of mine.  Two spectators each hold out any number of fingers on one of their hands.  (E.g. one might put out 3 fingers and the other, 2.)  A third spectator tells the magician, who is looking the other way, the total (in this example, 5).  The magician immediately announces that…

Not many tricks can be done over the phone.  This is one! You and your friend alternate turns, each selecting a number from 1-9.  The object of the game is to be the first to have 3 of your numbers total 15.  You have a secret strategy that lets you win much of the time.