I have made beasties with pupils aged from six up to 10. My Exeter University colleague John Twyford even sets it as a task for teacher trainees, to get them to think about designing and making things with children.
The idea is simplicity itself. Children first make a traditional "bobbin tank" from a cotton reel and an elastic band. Then they design a shell to put on top of it, a turtle or 'Nessie' perhaps, and hello beastie!
I usually start with a bit of science. How can you make a toy that moves? A quick demonstration of one or two clockwork and battery toys shows that some form of energy is necessary to make them toddle along.
Our muscles can be used to coil a spring or stretch and twist an elastic band. Energy thus stored can be released to drive movement. An old-fashioned watch is a good example of this: we wind it up and the energy is slowly released to turn the hands.
Next step is to make the beastie's "motor". Push the elastic band through the cotton reel and a piece of candle. The candle helps to stop all the energy being released at once, as it slows down the rotation of the reel, but the hole left by the removed wick usually needs to be widened with a thin file or a cocktail stick. Secure one end with a short piece of stick and use a larger stick for the other end of the elastic band.
How on earth do you actually get the elastic band through both the cotton reel and the piece of candle? With patience, is the answer - it is quite a challenge.
Some of the children may find this a frustrating exercise, as fingers drop bits and elastic refuses to obey, so calming help is often needed. A knot may have to be tied in the rubber band if it is too long, or it may have to be threaded through and then back again.
There are lots of fun questions to be answered, according to the age and ability of the children. How far will it run? They can measure the distance. Does a longer or shorter stick make a difference? (Yes, insert a shorter stick and the tank will run more quickly.) Try running your turtle on a carpet or a smooth table top and see how the two compare.
How many turns do you have to give the elastic to get the tank to travel a certain distance? In theory, the more you turn the more energy is available, but in practice the elastic band will eventually snap. Have a few spares ready for tearful junior scientists who turned the stick once too often.
Once the tank has been developed and tested we are ready for the beastie stage. Design your own creature or monster to put over the tank as a cover. The whole contraption will then move along as if by magic and you have made your very own moving toy.
There are no limits to the imagination, but the easiest starter is to make a turtle. Cut out a circle of coloured card, about 15 centimetres in diameter. Then cut a line from the edge to the middle and the circle can now be shaped and glued so that it resembles a dustbin lid. A head and tail can be added and the whole creature painted or decorated with felt-tips, a real design challenge.
Some children will want to be much more ambitious. They may want to design an insect with transparent wings (use acetate or cellophane), a spider, a tank, a multi-coloured whatsit, a space creature, an abstract shape, an animal, a human being even (difficult, but not impossible, easier on all fours or in a sitting position).
A glue gun is handy if children want to stick three-dimensional shapes to the card, as ordinary glue may not hold properly. Some three-dimensional additions can be very impressive, and studded shapes can look especially striking on the beastie's shell.
Inevitably they want to give their creature a name, parade it in front of their friends, have races against each other, take it home. Several together in a display can look quite spectacular, or even sinister and menacing sometimes.
Brave teachers will then line them all up and let children write a story about the day the beasties invaded their school.
"You should have seem Mrs Morgan's face when the Beasties from Hell marched into assembly one morning..." A strong stomach may be needed to read the rest.
Ted Wragg is professor of education at Exeter University