Completely Different Method - Suitable for Stage!
Show a large wooden baton with three cords running through it. At the end of each cord is a ball, two of which are of the same color (say red), and one a different color (say yellow).
Now show the cord run freely through the holes, and the balls are securely tied on the cords.
Explain you'll make the ball jump from one cord to the other. You pass the baton behind your back, and the odd colored ball jumps from one end (say left) to the other (right).
The audience is not impressed, - they know you are just turning the baton around behind your back.
You appear offended at not be believed, and offer to prove conclusively that the balls do jump from one cord to the other. The baton is passed behind the back once again, and when brought to the front, the odd colored ball is now tied on the middle rope, between the two similar colored ones.
Thick cords, and large balls, making it very visible even on the larger stage.
Easily adapt the baton for use in any of the other presentations!