BS's class has been reading about Newton's Laws of Motion in science. To understand the third law of motion, "Every action has an equal and opposite reaction", we found this simple and fun experiment from where else but the world wide web. Details on the experiment can be found here at NASA.
What you Need
1 Bendy Straw
1 Pencil with eraser at the back
What You do
Stretch the balloon a bit and then fit the nozzle of the balloon over the non-bendy end of the straw. Secure the balloon edge to the straw with tape.
Now find the balancing point of the straw with the balloon attached. You can do this by trying to balance it on your finger.
Once you have found the balance point(which will be the pivot), put the pin through the straw and lodge it on the eraser side of the pencil
Now blow through the open end of the straw to inflate the balloon. Once the balloon is inflated move your mouth back and let go of the straw.
See, what happens.
Newton's Third Law of Motion says, every action is, accompanied by an opposite and equal reaction. In this case, the balloon produces an action by squeezing out the air inside causing the air to rush out the straw. So the air traveling out is the action. The air, traveling around the bend in the straw, imparts a reaction force at a right angle to the straw. The result is that the balloon and straw spins around the pin.
This YouTube video will give you a clearer idea of what is being done.