Here’s a great demonstration/mad party trick that’ll go down a storm at the end of the exam season. Or even before then, by way of light relief. The main thing you need is one of those garden leaf-blowers, and a spare toilet roll or two …

Jonathan Sanderson, our favourite kids’ science tv producer, is here to explain how it’s done. And he says it good for the leaf blowers’ mental health too …

“High summer is a difficult time of year for leaf blowers. With no leaves to blow, they tend to feel useless, dejected, even depressed. Here's a way of cheering yours up, and putting meaning back into its plastic life.

What you need

• A leaf blower
• Some toilet rolls
• The cardboard tube from a roll of cling-film
• The end of a cardboard box
• Some tape

What you do

1. Cut a disc out of the cardboard box, larger in diameter than your toilet rolls, and cut a hole in the middle of this just large enough to pass the cardboard tube through. Do that, and tape the disc in place, so you've made what looks a bit like a very short stubby sword with a handguard. We'll call it a 'handle.'
2. Hold the handle with one hand, and put a toilet roll on the end your hand isn't holding. If you're doing this right, the guard should be between your hand and the toilet roll. Tilt the whole thing so the toilet roll is leaning into the guard, and finally make sure the loose flap of toilet paper is draped over the top of the roll, facing away from you.
3. Now, aim the leafblower high, turn it on, and very carefully bring the toilet roll up so it's just beneath the flow of air.
4. You should find that the airflow spools the toilet paper off the roll, sending it flying high in the air and emptying the roll in seconds. This is quite (a) funny, (b) spectacular and (c) ecologically unsound – so we’re not recommending you do it toooo often. But it’s fun while it lasts.

What's going on

The fast-moving jet of air from the leafblower is at a lower pressure than the air around it. So, when you bring the toilet paper close, the high-pressure air beneath it lifts the paper and shoves it into the airstream. It's accelerated, of course, and that sends the arc of paper spooling out across your room/playground/garden/mesmerised dog/etc.

It's a wonderfully fun trick - just make sure the cardboard disc guards your hand from the fast-spinning toilet roll. It's  possible to collect a nasty friction burn if you're not careful. Believe me, I know.

Also, do think about what happens to the paper afterward. The experiment is a heap of fun, the science is solid... but the aftermath is messy.

Hats stay firmly on for this week's scientific 'wrong idea' which concerned bumpy heads - and the quest for easy answers and easy money in 19th C psychology.

Ian Francis has been investigating ...

'Phrenology' was the name given to a practice that claimed to be able to figure out a person's personality by simply feeling the shape of the subject's head. Various character traits were supposed to have different 'organs' within the brain, which were better developed (i.e. bigger) in some individuals than others and detectable by appropriately trained fingers working over the skull. A poet would therefore have a bigger poetry organ, a shifty type would have a decidedly well-developed criminality organ … and the organ for 'cunning' was presumably bigger than average in pay-per-reading phrenologists.

We have the German Franz Joseph Gall to thank for this pseudo-science, although in much of continental Europe his wrong idea was a minority taste. It was however enthusiastically adopted by rip-off merchants everywhere and by large numbers of well-to-do Brits and Americans. There's a dodgy historical subtext at work, too, as phrenology was used to justify poor treatment of 'lower' classes, races of people and ‘born criminals’. As with all forms of medical quackery, phrenology never had any scientific basis, as its followers only accepted evidence as valid if it backed up their existing ideas. Its practitioners consequently had to fight off attacks from many quarters including the scientific establishment, the press, and the Church.

F J Gall doesn't make many dictionaries of scientists for the right reasons, although he did contribute to our knowledge of brain structure. And to be fair, there's also an element of sense in his idea of 'mini-organs' in the brain doing certain jobs. Certain areas of the brain do have specific functions, but we can say for sure that feeling the skull's lumps and bumps tells you nothing of the character of the person whose brain is within, any more than feeling a rigid shoe box can tell you whether it's a pair of sporty Reeboks, flip flops or fancy Manolo Blahniks inside.

For more on phrenology including neat pics of phrenologically-labelled skulls, click the link.

You can take your hat back off now!