Tear a sheet of paper almost into 3 parts as shown. The paper should hang together at the corners by the merest possible thread. Hold A in one hand, C in the other hand, and try to jerk them apart so that all pieces separate. It's not possible! The paper will break at one spot, not at the other. Let several students "try" to break the pieces. Then you try your "special sheet". You can cheat by using tape to attach a quarter to the top of B in the back, unseen. This increases B's inertia enough to cause the paper to break at BOTH spots.