palloncino a reazione

a jet baloon
1. a string
2.a stiky tape
3. a baloon
4. a straw
1. fit the string into the straw
2. tie one end of the string to a door handle.
3. fix the other end of the string to another handle.
4. make sure the string is taut
5. blow up the baloon
6. keep the baloon closed with your fingers
7. fix  the baloon to the straw
8. put the straw with the baloon to one end of the string with the back towards the rest of the string
8. open your fingers and set the baloon free
what can you observe?
the baloon runs away very fast on the string
why does it happen?
when the baloon is closed, the air inside pushes against the side of the baloon
when we open the baloon the air comes out pushing against the sides of the baloon and (as reaction) the baloon moves very quikly.