The Drinking Straw Propeller

What you need:

Two drinking straws, bendy.
A pair of scissors, not bendy.

What to do:

Take one of the straws, and cut off the long end so you have about four centimetres of tubing on either side of the bendy bit.
Now bend the straw into a right-angle. Repeat the process with the other straw, but cut a slit in one end of it. Roll that end up, and slide it into the first straw to form a joint.
Arrange them so you have a 'U' shape lying flat on a table, then rotate the joint so one end points straight upwards. Take a look at the picture if that sounds confusing - it's really very easy.
Place one end of the straw contraption lightly in your mouth, and blow. With a little practice, it's possible to make the thing whizz around in a pleasingly silly manner, like some demented drinking straw propeller.

What's going on?

As the air blasts out it thrusts the straw around - action and reaction, Newton two, and all that. It's obvious, right?

Well, yes. But alas, it pains me to point out the complexity of the situation: When you suck, the propeller stubbornly refuses to spin the other way. Or, indeed, at all. Why?

Therein lies a tale, for this very problem has challenged some of the greatest minds of our time, most notably the Nobel Prize-winning physicist and bongo-player Richard Feynman. In such esteemed company I'm slightly nervous about giving a definite answer. Or, indeed, any at all. But we're all friends here so I'll wade in regardless.

Sucking on the straw is not the exact opposite of blowing down it. This is a common feature of aerodynamics - it's easy to blow out a candle, but sucking one out doesn't work nearly so well. And while you see many jet-propelled aircraft, I've yet to come across one that's powered by vacuum cleaners.

The difference is that when air is forced out of a nozzle, it's all travelling in roughly the same direction, so it comes out as a narrow jet. But when air is sucked into that same nozzle, it comes from all around the opening - not just directly in front, where the jet was, but from the sides, the top and bottom, and even behind the nozzle, along the tubing.

So while the jet is pushing against the air outside - and you can feel that reaction force as thrust - the sucked air is coming from all different directions, and there's no overall force.