# Puzzles

## Raketti

This wooden toy has been around for 20 to 30 years. Some versions have been made in plastic, although the wooden version is more traditional.

Raketti consists of a wooden block with a cylindrical hole drilled into it, and a wooden cylinder that fits loosely into the hole. You present it as a challenge to someone - get the central cylinder out of the hole without touching the toy at all. You cannot even touch the table!

It seems impossible... until you see it done. You blow sharply over the top of the cylinder, which pops smartly up into the air. We used to think it was all to do with moving airstreams and the Bernoulli effect, although now we think it is probably an example of an air piston.

Watch the video...

Dr Iain Dupere from the University of Manchester has kindly sent us a more complete explanation.

"When blowing over the top, because it is impossible to blow exactly symmetrically, the flow is forced to swirl around the cone on top of the plug. In doing so, it clings to the cone and is pushed down because it sticks to the downward shape on the back slightly. This downward motion requires a downward force by the plug, whose reaction results in an upwards force on the plug (this is classical lift like on an aircraft). The air which is pushed into the box has nowhere to go when it gets to the bottom other than underneath the plug where it pushes it up like a rocket (or an air piston). The combination of these two effects results in the high velocity with which the plug leaves the box."