When thinking about the properties of different materials it is common to ask children to match properties with appropriate uses. It would be a rare child who would champion a wooden saucepan with a metal handle or a toaster constructed of chocolate.
The apparent absurdity of such choices can stimulate lively discussions on what we really know about material properties. It is always good to introduce a little philosophical doubt into these discussions, looking back to when science was natural philosophy.
Fold two pieces of A4 paper into a shallow dish-shape. Ask what will happen if a paper dish is held above a candle flame. Next, add some water, about 1cm deep, to one of the dishes and suspend it over the flame. Ensure the flame is central and only heats paper which has water above it. The children may be surprised to find that paper saucepans work, the water heats without the paper igniting.
Repeat with the second paper dish, this time empty. Of course, it ignites immediately. Children should realise that the temperature of the paper never rises high enough to ignite so long as there is water present. The heat is conducted away to the water, preventing a fire.
Now try a hotter flame, such as a gas burner, using a dish made from thin aluminium cooking foil. The dish melts and shrivels. Contrast this with a similar metal dish containing some water, as before.
The experiment can be extended to a consideration of what is happening when a liquid boils. Show them a beaker of water containing a thermometer. As it heats up, the temperature initially rises every minute but becomes constant as it reaches the boiling point of the water.
Ask where the heat is going now that the temperature has stopped rising.
During a change of state the temperature stays the same, the heat is used to turn liquid into vapour.
Finish off by asking for crazy uses of materials, each must have a reason for its absurdity.
Ray Oliver teaches science at St Albans Girls' School, Hertfordshire