Stardate Friday 31 March 2006 Issue 176

“Simple is good. Simple ingredients, simple tricks, simple explanations - anything that saves one from having to confront the complex. It's a pity, then, that even something as seemingly trivial as a pair of drinking straws can turn out to be brain-numbingly hard to explain.

What you need:

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

What to do:  

1. 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.
2. 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.
3. 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.
4. 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.

That's my explanation, anyway. What's yours?”

“You will probably have heard of amino acids - the building blocks of protein. They generally get a good press; some are essential in the human diet because the body can’t make them, while others can be made from other nutrients we eat. But did you know that there are also some that are positively havoc-wreaking? They are referred to as ‘non-protein amino acids’ (NPAs). There are thousands of these substances, around 300 of which are found in plants. Canavanine is one of these.

Canavanine’s role in nature is to protect plants and their seeds by acting as a natural insecticide. It can also kill rodents and, potentially, us. It’s found in legumes, particularly the jack bean (Canavalia ensiformis), and, to a lesser degree, alfalfa (Medicago sativa). Both of these are used extensively for animal feed and also for human consumption, although the jack bean is predominantly grown and used in the tropics. Alfalfa, on the other hand, is easy to find in UK health food shops, either as dried seeds or ready-sprouted. The latter makes a highly nutritious addition to salads. But as with many foods, too much of a good thing can turn out to be not such a good thing after all, so go easy …

Canavanine can cause autoimmune haemolytic anaemia, a condition in which the body's immune system destroys its own blood cells. Canavanine is also believed to worsen, or possibly even trigger, a condition called Systemic Lupus Erythematosus (SLE) or “lupus”. This is another autoimmune condition, where the body attacks its own tissue and organs, such as the kidneys, the heart, the brain, the joints and the skin. So, far, there is no cure for it.

How does Canavanine come to cause so much trouble? Well, first it fools the body by pretending to be something else. Canavanine is an arginine analogue, which means that it looks almost exactly like the non-essential amino acid arginine. Arginine is widely used by the body, but when canavanine is present, this alien substance is integrated into human proteins (such as enzymes) instead. This leaves them unable to carry out their intended functions, causing many biochemical and metabolic disturbances. How exactly this may lead to autoimmune diseases is not yet known.