Inflate the two balloons and tie them closed, then use the string
to hang them from opposite ends of the yardstick. balance the
yardstick on your finger. Now pop one of the balloons.
The yardstick becomes unbalanced as the popped balloon loses the mass of air
it held. The two rubber balloons still weigh the same, but the
inflated one now holds more air, which makes that end of the
scale heavier, so it sinks.