First, make a single pulse: Have your partner hold one end of the phone cord or attach it firmly in place at about waist height. Stretch the cord so it’s about 12 ft (4 m) from you to your partner, and then jerk the hand holding the cord to the right about one hand-span and immediately back again to set a traveling pulse in motion. Notice that a rightward pulse travels down the cord, but a leftward pulse returns after bouncing off the far end of the cord.
If you have trouble seeing that the pulse is reversed when it is reflected, set the cord on the ground and stretch it to about 12 ft (4 m). Mark a straight line on the ground under the cord using masking tape. The direction of motion of the pulses will be much more visible when compared to the reference tape.
If it isn’t there already, set the cord on the ground and stretch it to 12 ft (4 m). Place an empty paper or plastic foam cup next to the center of the cord. Once again, send a single pulse down the cord, making sure you move your hand to the side the cup is on when you make the pulse. It should hit the cup and knock it over.
Now make two pulses and add them together: This time, both you and your partner will set pulses in motion.
Keep the cord on the ground as it was before, stretched between you and your partner. Then set up the cup again at the center of the stretched cord. First, you and your partner should both send waves down the cord. Both pulses should be toward the side the cup is on. Notice that when the pulses meet in the middle, you end up with an even bigger wave that knocks down the cup. Carefully time your pulses so that they both arrive at the center of the cord at the same time.
Set up the cup again.
This time, you and your partner should send waves through the cord in opposite directions: While you send a pulse on the same side as the cup, have your partner send an identical pulse on the side away from the cup. The pulses will arrive at the center at the same time. The rightward pulse will add to the leftward pulse and cancel it out. The sum of the waves will be zero. The waves will miss the cup, and the cup will remain standing.