Did any of your pupils ask how Santa got down the chimney? Try this experiment and all will be revealed.

Make your Santa from a red balloon that is partly inflated. Decorate your balloon with a marker pen to make it look Santa-like. A shelled hard-boiled egg works equally well, although it is more difficult to decorate.

Now make your chimney. Take a glass bottle with a reasonably wide neck. If you are using an egg the neck should be about 1cm narrower than the width of the egg. The bottle needs to accommodate at least 0.5 litres and the neck needs to be narrower than the body of the bottle. The neck also needs to be smooth, as any jagged edges will damage Santa. (An old fashioned milk bottle works brilliantly.) To make the chimney look authentic you could edge the top with card, decorated to look like bricks. Make sure you cover only the first few centimetres of bottle so you can see what is going on.

Now create your fire. Light three matches and drop them into the bottle. (Alternatively, you could light a small piece of crumpled paper and drop that into the bottle.) Immediately place your Santa on top of the bottle, and wait. You should have the satisfaction of seeing your Santa jiggle up and down and then, once the flame has gone out, being forced through the bottle opening and into the bottle.

It goes without saying that you should take extra care when using matches, glass bottles, and heat. Be careful to conduct your experiment in a sink, with plenty of water on hand. During the experiment the air pressure inside the bottle decreases, and the higher pressure outside the bottle forces Santa into the chimney. If you have used an egg, you can remove Santa by blowing into the bottle, increasing the air pressure and forcing Santa out.

Sophie Duncan is a physicist and programme manager with Planet Science (formerly Science Year) www.planet-science.com