Grouping materials by their properties
Chemistry · Key Stage 2 · The School
Sort by a testable property
Materials can be grouped by hardness, flexibility, transparency, whether they conduct electricity or heat, whether they float, and whether they let water through. Each of those can be TESTED, so two people sorting the same objects agree. Grouping by "nice" or "useful" produces two different piles and teaches nothing.
One object, several groups
A metal spoon is hard, shiny, opaque, waterproof and conducts heat. It belongs in all of those groups at once, and which one matters depends on the question you are asking. Sorting is not about finding the one true box for an object; it is about choosing the property that matters for what you are doing.
Choosing for the job
Once you can name properties, choosing a material becomes reasoning rather than guessing. A window needs transparency and rigidity. A saucepan handle needs to NOT conduct heat. A raincoat needs to be flexible and waterproof. Ask what the object must survive and what it must do, and the shortlist appears on its own.
Take a copper coin, a plastic ruler, a glass marble, a wooden peg and a steel key. Sort by CONDUCTS ELECTRICITY: coin and key in one pile, the rest in the other. Now mix them up and sort by TRANSPARENT: only the marble. Now by HARD: coin, marble, key, and arguably the ruler. Three sorts, three different sets of piles, same five objects — which is why you state the property before you start.