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Air-cushion card

This is less of a craft idea and more of a physical experiment.

Instructions

Out of strong card cut out a circle (approx. 9cm)
Separate apart a toilet roll tube in the middle and then roll it around e.g. a thick pen, sticking together the new roll with tape.
Draw the circumference of this tube onto the card.
Cut an ‘X’ into the circle and bend the ‘edges’ upwards.
Stick the tube onto the card.
Place the pen into the tube so that the bent up edges also stick to the tube.

How to start?

Inflate a balloon, hold the end and place it over the card.
Put the balloon over the card and inflate it through the card.
Let go of the air cushion and observe.

Why does this happen?

You are familiar with the so called “hydrodynamic paradox”. This implies that in the narrow section, the pressure flow is reduced. So instead of just pushing the balloon away, a sub pressure builds up under the card and because of this the balloon floats and doesn’t fly away.