The best suggestion went even further than this and showed that particles moved faster if the temperature was higher.
* Take two similarly sized and shaped beakers or large jam jars.
* Fill one with cold water and one with warm water.
* Put a wide straw into each container.
* Two students each drop a coloured crystal into a straw at the same time.
We used potassium permanganate crystals, which are bright purple. You could use small coloured bath crystals. The purpose of the straw is to place the crystal at the bottom of the container without swirling the water.
* Gently remove the straw.
* Start a stopwatch and observe.
Although the water has not been disturbed by your action, the coloured crystals create swirling patterns in the liquid.
This is evidence that the particles in the crystals do indeed move. The warm water is seen to increase the speed of the diffusion as the purple swirls are even more pronounced.
In our view this beats the textbook demonstration that has to be done by the teacher - smelly brown bromine gas in a fume cupboard.
Science teacher, Hitchin Girls' School, Herts