Looking for a really flexible - and free - device that can be used in numerous ways at every key stage?

Let me introduce the humble film canister, the small black or clear plastic container with a lid that you (or your film processing company) throw away.

I know of numerous uses for the film canister and have even invented a few of my own.

Insulation experiment

Fill some canisters two-thirds full with water. Measure this carefully so that all the canisters contain exactly the same. Put them in the freezer compartment of the fridge without the lids on. When frozen, put the lids on and take them out just before you need them. Provide the children with a range of insulating materials for their canister. A good selection would be cardboard, bubble-wrap, wool, aluminium foil and cotton wool. A roll of sticky tape is also useful. Wrap the canisters and leave for about half an hour at room temperature, then pour the contents of each through a sieve into a measuring cylinder. This will show how much has melted as the sieve will catch the remaining ice.

Magic pennies

Put three pennies, one (and only one) of which is dated before 1992, into identical film canisters. Your trick is to reveal which canister contains the oldest coin without looking at them. Provide red herrings - sniff each canister in turn, drop them, shake them or try to peer through an imaginary hole. Meanwhile, hold a magnet beneath each one. The oldest penny is not attracted to the magnet, as it is made from bronze rather than copperplated steel. Challenge children to work out how the trick works.

The Moon

Drill or bore a small hole about 5mm diameter halfway down a black film canister. Inside the canister, directly opposite the hole, stick a white bead or polystyrene ball. Double-sided sticky tape is good for this. Put the lid on the canister. Ask the children to look through the small hole.

They should not be able to see anything. Then take the lid off. The bead is now visible through the hole - like the Moon, it is only visible when reflecting light from another source.

Alka-Seltzer rocket

This trick has been performed (rather badly) by Jeremy Clarkson on television. Put a small amount of water in the bottom of a canister and check that the lid fits well as any leak ruins the trick. Pop in one Alka-Seltzer, put the lid on quickly and turn upside down. The canister should rocket up to 10 metres into the air. This experiment is probably better done as a teacher demonstration than a pupil activity. Aless spectacular version involves leaving the canister the right way up and letting the lid blow off.

Dried peas

Fill a film canister with dried peas and add water until it overflows. Put the lid on. Leave for a few hours and the peas should swell enough to force the lid off.

Water to ice

Fill a canister with water. Put the lid on and put in the freezer compartment of the fridge. As the water turns to ice it expands and should force the lid off.

Smells

Film canisters are very good for experiments with smells. Put the smell on a piece of paper or cloth and put it in the film canister. Put the lid on and make a few holes in it with a drawing pin then sniff.

Identifying materials

Fill some canisters with a range of materials, such as rice, flour, dried peas, water, salt, sand and sawdust. Then shake them - can you identify the contents?

Other ideas

* Stick several canisters to a piece of plastic to make a test-tube holder.

* Use as containers for specimens.

* Fill with sand to make weights.

* Make wheels for buggies.

So next time you take your film in to be developed, take a large bag and ask for their discarded canisters.

Mike Dennis is primary science co-ordinator at the Oxford Trust www.oxtrust.org.uk