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Have you got the bottle?

John Stinger mixes a lesson that will bring a splash of colour to your classroom

Take an empty fizzy-drink bottle, water and a little food colouring, and you have all the makings of a stimulating experience. Infants will sit spellbound by it and come up with vivid descriptive vocabulary. Juniors are inspired to tell and write stories. Teachers are entranced, and moved to write poetry.

All have been watching colours weave and uncoil in an old lemonade bottle. For similar results, take a two-litre bottle, remove the label and fill it with water. Put it in front of a sheet of white paper for the best effect. Drop a couple of drops of food colour in from a dropper or a drinking straw. Screw down the lid to prevent spills - and watch.

The colours spiral down in intricate patterns. They trail smoke like acrobatic aircraft. Particles climb inexplicably, only to sink into the billowing clouds. It's a bit like a lava lamp but much more beautiful.

What's happening? Children speculate about it. Putting two identical bottles next to each other makes comparison by observation easy. They have found by investigation that the colours behave differently in warm water and cold; the colours seem to sink and spiral faster in cold water. It matters whether you fill the bottle and use it immediately, or let it stand - you get more delicate patterns with still water. They've explored different colours, but found it difficult to ensure equal amounts of colour, and so the tests were not fair and the outcomes were uncertain.

This is helpful as an introduction to the behaviour of small particles. What causes the movement? Whether it's the movement of the water in the bottle, the different temperatures of food colour and water, or whether it's the effect of something more complex is not clear. Brownian motion - the random movement of tiny particles - was first observed by Robert Brown, a Scottish botanist, in 1827 and explained by Albert Einstein in 1905. Perhaps moving particles of water are pushing the food colour around.

It's this movement that accounts for the way gases diffuse through a room - try opening an air-freshener in one corner and ask children to lift their hands when they can smell it. Why should it fill the room in this way? It is because the tiny particles of gases are constantly moving. Sooner or later, the gas will enter all the space available.

You don't have to see the experience as purely scientific. Like so much of science, it is also aesthetically pleasing. It may inspire gruesome stories - "clouds of blood", "underwater explosion" - or more touching outcomes - "delicate tracery", "soft clouds". It may even, as one teacher found to her embarrassment, be so relaxing that it will put you to sleep.

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