Here's how it works:

1. You blow air into the balloon. This forces air molecules inside the balloon, increasing the number of molecules and their movement.

2. Increased pressure inside. The increased number of air molecules and their movement creates higher pressure inside the balloon compared to the air pressure outside.

3. Balloon stretches. The higher pressure inside the balloon pushes outwards against the elastic material of the balloon, causing it to stretch.

4. Equilibrium. The balloon continues to inflate until the pressure inside the balloon equals the pressure outside, creating a balance.

So, it's not a specific force like "blowing force" or "stretching force," but rather the difference in air pressure between the inside and outside of the balloon that causes it to expand.