You only need four things:
a push-up water bottle top
Superglue the water bottle top to the CD right over its center opening to make a valve. If you don't have a push-up water bottle top you can always use one from an old bottle of dishwashing detergent.
After the glue is dry and with the valve in the closed position, blow up the balloon and fix it over the valve in the middle of the CD.
Place the hovercraft on a hard and flat floor (not carpet), let go of the balloon and pull the bottle valve to the open position. Watch it hover over the floor as the air escapes from the balloon through the valve.
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