gravity is what causes air pressure..just like it causes densitly of things and just like it has the pull on the earth,other planets,and all stars
Because there is lots of air molecules put in by a pump, Which forces them to go into the tyre. All of the air molecules get squashed together which makes it tighter and tighter which causes the tire...
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the amount of force presses on a certain area is known as pressure so when you blow air in to the balloon your giving pressure on a certain place.