Book page 896 Updated on2026-09-05
The two balloons move towards each other and the gap between them closes, even though you are blowing air into that gap.
Two equally sized inflated balloons hang from a stick with a gap of 6–10 cm between them. Most people expect the blast of air to push them apart. Instead, the moment you blow into the narrow space, the balloons swing inwards and may even touch.
Blowing harder makes the balloons come together faster and more strongly.
The harder you blow, the higher the speed of the air rushing between the balloons, and the lower the pressure it creates there. The difference between the outside pressure and the pressure in the gap therefore grows, so the inward push on each balloon grows too, and they approach each other more quickly.
That high-speed winds are accompanied by a reduced air pressure.
Blowing between the balloons creates a region of fast-moving air, and therefore a region of low pressure, in the gap. The air surrounding the balloons on their outer sides is still at ordinary atmospheric pressure, which is now higher. That higher pressure pushes each balloon inwards, towards the low-pressure region — so they meet in the middle.