Air pressure is the force exerted by the weight of the air above a given area. The air inside a balloon is pushing against the inside surface of the balloon in all directions. This pressure is equal to the weight of the column of air above the balloon. The higher the balloon goes, the less air is above it, and the less the pressure. This is why balloons float up until they reach a certain altitude where the air pressure is equal to the pressure inside the balloon.

Inside the balloon, air molecules are constantly colliding with the balloon's surface. The force of these collisions creates pressure. The number of collisions per second and the force of each collision determine the pressure exerted by the air particles. When you blow air into a balloon, you increase the number of air molecules inside and the force of their collisions, which increases the air pressure inside the balloon.

Here's a simplified explanation:

1. Imagine a balloon floating in the air. The air above the balloon is pressing down on it, creating a force called air pressure.

2. Inside the balloon, air molecules are moving randomly and colliding with each other and with the inner surface of the balloon.

3. When air molecules collide with the balloon's surface, they exert a tiny force, similar to when someone gently taps your hand.

4. Millions of these collisions occur every second, and the collective effect of these tiny forces creates the air pressure inside the balloon.

5. As more air is added to the balloon, the number of air molecules and their collisions increase, resulting in a higher air pressure inside.

The pressure exerted by the air molecules inside the balloon pushes against the balloon's elastic material, causing it to expand and take on a round shape.