Balance the meter stick ASYMMETRICALLY on the two rotators, and start the rotators in motion. Then balance the meter stick as close to SYMMETRICALLY as you can and start the rotators. It will always fall off if the rotators rotate outward, and will always end up balanced with the CM in the center if the rotators rotate inward. Which direction it moves depends on very small asymmetries!! Click the links below for several cases:
Try different starting points. Try adding a 100 gram weight onto either end. (Please ask for the weight so we do not overlook it.) It always works. Ask your students what they think will happen before doing the original experiment. Ask them again after adding a weight to one side.