What you Need
What to do
Cut off the long end of one of the straws so it is about the same length as the short side.
Fold the short end of the long straw slightly and then push it into one of the ends of the short straw.
Put the long straw in your mouth, and bend the first joint to point downwards, and then the second so it points to the left or the right.
Try to hold the straw in your lips gently and blow out of it. Try to stop it coming out of your mouth too much without stopping it spinning. What happens?
Now try sucking. What happens?
What may happen
You should find that the rotor spins around really quite fast when you blow, but nothing happens when you suck.
Why does it happen?
When you blow through the straw the air leaves all in the same direction, in the form of a jet. The straw channels the air in one direction and, according to Newton's laws, the air pushes back so the rotor is pushed around.
When you suck the air coming in is coming from all directions so it doesn't carry any momentum so it has none to transfer to the straw, and it isn't pushed around.