We made a balloon rocket!
We attached a balloon to a piece of string and let the balloon go. It was so much fun watching it whizz down the string to the other end. 
We had to change our design a little bit because sometimes it didn't work very well. 
We discovered that we needed to put quite big pieces if selotape onto the balloon or else the selotape would just come off. 
We found out that the air in the balloon created a force which pushed the balloon along the piece of string like a rocket.