You explain that you like numbers and that each person can be associated with a particular number. You look at a spectator and name a number, say "27". Now you fan through the cards face up and have them select one when they see something they like. You then turn the packet face down and show that you have given each card a different number. You ask they spectator to recall the number you assigned to them. They turn over their freely selected card and it has that number written on the back.