On those rare British sunny days, people sometimes pull curtains to prevent the fading of carpets and furniture. If there is a small gap between the curtains, something unusual can be observed. In the concentrated light of the sunbeam, you can see dust particles illuminated in the air. The small particles, usually invisible, scatter light and so become visible.

This property can be used to investigate colloids. In a true solution, like salt water, the particles are extremely small and impossible to see, even with a microscope.

In a mixture of water with much larger particles, such as sand, if the particles are stirred round they soon settle on the base of the container when the water is still. Colloids are in between such mixtures and true solutions. The particles stay suspended in a liquid or gas, not being heavy enough to sink or small enough for the mixture to be counted as a true solution.

One familiar example of a colloid is fog. Fogs form when water drops condense around dust or other small particles in the air. At the seaside, thick sea fogs form around minute salt particles above the ocean surface.

Car headlights produce a clearly defined beam in fog, like a lighthouse beam.

Get the children to produce a range of colloids and investigate them with light beams. In a darkened room, try shining a bright projector beam or torch through a spray of aerosol. You could use air freshener or perfume to produce a fine mist in the room.

Pupils can also produce colloids in water. Use tall glass containers and try stirring in small amounts of materials that produce colloids. Try clay, gelatine or liquid glue. Put the glass container in the light beam and move it slowly up and down. Look for a clearly defined bright zone in the liquid, just like the effect of car headlights in a severe fog. Investigate which materials produce the clearest beam, by identifying those that reflect light most effectively.

Children can produce colloids that magically appear in front of their eyes.

Put a weak solution of sodium thiosulphate into a glass jar and shine a light beam through it. The beam is not clearly visible.

Now add a few millilitres of any dilute acid and wait. Within minutes a colloid of sulphur appears in the jar, pale yellow and clearly defined.