A balloon rocket is a balloon filled with air. Besides being simple toys, balloon rockets are a widely used teaching device to demonstrate physical principles and the functioning of a rocket.
To launch a simple rocket, a person releases the opening of a balloon, which is then propelled somewhat randomly by the escape of the air which creates thrust. The flight altitude amounts to some meters. The balloon rocket can be used easily to demonstrate simple physics, namely Newton’s third law.
A common variant of the balloon rocket consists in adding other components such as a string, a drinking straw and adhesive tape to the balloon itself. The string is threaded through the straw and is attached at both ends to objects of some kind, such as a doorknob on one end and a chair on the other. The straw is then taped to the side of the air-filled balloon, with the open end of the balloon touching one of the objects. When the balloon is released, the thrust from the opening propels it along the length of the string. Alternatively, a balloon rocket car can be built
The balloon can also be filled with gases other than air, with similar results.
Source: Wikipedia (All text is available under the terms of the GNU Free Documentation License)
For More Information: NASA Office of Education. Rockets: A Teacher’s Guide with Activities in Science, Mathematics, and Technology.