A least one balloon
To make your rocket:
String the straw through whatever kind of line you're using, making sure the straw can move easily.
Secure each end of your line to something (trees, chairs...whatever you can).
Get a piece or two of tape ready.
Blow up your balloon and pinch the end shut.
Tape it to the straw.
When you're ready, let go of the balloon and watch it zoom down your line!
It was quite difficult to take pictures of this in action, but here you can see the balloon whizzing past an amazed Ty!
We did this over and over for half an hour per Tyler's request, using a few different balloons as they wore out from time to time. Tyler's job was so push the string back and to bring the balloon back as it often fell off at the end.
Near the end, Tyler asked to do two balloons at once. Wow. It's a bit tricky enough to do this with one balloon, but two?! I thought I'd need four hands but I actually succeeded in the double balloon rocket three times (Mommies can do anything!). It was neat to see the rocket zoom with double the thrust! I don't think he knew it would do this, but we did talk simply about how the air coming out of the balloon was pushing it down the line.
I also tried to blow up the balloon to different levels so we could observe how far it would go based on how much or little air was in it, but Ty wasn't having it. He wanted the balloon to go at full force! He was also tired and kinda on the edge of meltdown (this is a common theme lately around here unfortunately) so I didn't push it. There's time to try that another day. In the future I'd like to also put the line at different inclines and use different shaped balloons!
I knew Tyler would want to watch the balloon in action later when it was taken down, so I took video. I figured since it's difficult to see what's going on from pictures I'd share it here too. Enjoy!