Question

why is it difficult to force the paper ball into the bottle? By blowing into the bottle ​

Answer 1

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➪ This is because air occupies space. The ball will not go in by blowing because the remaining space in the bottle is already used up by the air, and there is no more space left for the ball to fit.

Answer 2

Bernoulli noticed that air has a lower pressure when it's moving more quickly. As you blow into the mouth of the bottle the air is moving fastest - so is at the lowest pressure. Inside the bottle, the air is moving more slowly - so a higher pressure. As the pressure is higher inside the bottle than in the mouth this pushes the paper out!