Pass the string through a hallowplastic straw.
Tying string with Supports
After that, tie one end of the string with a chair or some support, and the other end to another support in a way that the string seems like a bridge between the two objects.
Filling the Balloon
Blow the balloon with air and once it is full, pinch the neck firmly between your thumb and finger to stop the air in it. You can tie the neck with a piece of string but not very tightly.
Fixing the balloon with string
Now attach the balloon with the straw such that it is parallel to the string.
Releasing the Balloon
Now release the neck of balloon. You will notice that as the air blows out of the balloon, it moves along the straw in a straight motion like a rocket. As you try the same procedure with balloons of different sizes and strings cut at different lengths, you can prove that the distance traveled by a balloon depends on the volume of air it contains. Hence, the more air a balloon will have, the greater distance it will travel. And it will always move in the opposite direction of where the air is blowing out.