A spectator signs a freely selected card and draws a mouse on its face. She shuffles the cards, while you present the mousetrap. You  place the armed trap on her outstretched hand, and stroking one card at a time over the trap, you wait for the trap to go off. Of course, the card that is caught is the signed card with the mouse drawing! Use your own deck and pen.

This trick has it all! 

Comedy, Suspense, Audience Participation. Happens in their hands. Can be done close-up, stand-up or on stage. Packs small - plays big. A very, very commercial routine. And sooo easy to do.

You can also lay the trap on the table, and no fingers will get caught! Not as much fun, though.