An often used way to keep air inside the balloon ;-)
Balloons needed:
Any balloon of course
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1 Hold the lip of the balloon between your thumb, middle and index finger.
▶ video
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2 Stretch the lip around your finger.
▶ video
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3 Push the lip between the loop you just created and pull.
▶ video
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4 An airtight knot. There is also a video demonstrating it.
▶ video