Card pips change and multiply, driving your audience crazy!
Show a large card having one pip (e.g., diamond) on one side, and 4 pips (diamonds) on the other. Turning them over again reveals 3 pips on one side and 6 pips on the other!!
Let the audience in on "the secret." Once you've explained what you've actually been doing, they think they're "in the know." WRONG! You start showing the sides over and over again, with the pips multiplying beyond control! You hadn't really divulged "the secret" at all!
They're dumbfounded! Stupendous magic!