Take "big", and its noun "size" - your size is how big you are.
That's no news to you or to any key stage 3 pupil, but what you may not realise is how far this pattern extends through your vocabulary, or how useful it is in writing.
The noun "size" is the name for a scale on which one end is called "big" and the other "little".
There's no recognised word for nouns like "size", so you and your class can invent one - being rather mundane, we can't do better than "scale-nouns".
Scale-nouns often work like "size", with two opposed adjectives forming a little trio of closely related words: Size: - big - little
Weight: - heavy - light
Price: - expensive - cheap
And so on, through dozens of other trios. This is well worth exploring in class - every child should be able to think of some. We'll explain below why we think this is a useful exercise.
One interesting thing about these trios is how little help we get from the words themselves. If you didn't know its meaning, you could look at "size" all day and not guess that it had anything to do with "big" and "little".
But you do know its meaning, and the special relation is crystal clear. We give you "age", and you can immediately give us back the rest of the trio, even though they don't look (or sound) at all alike.
This is one of the reasons why we find trio-hunting is a good activity for key stage 3: it's an exercise in bringing meaning itself into focus.
To be honest we've cheated a bit by choosing untypical examples. Ok, "size" doesn't look or sound as though it belongs to its adjectives, but most scale-nouns do. "Height" sounds like "high" and "length" like "long". On the other hand, "height" is nothing like its other adjective, "low", nor is "length" like "short". Here are a couple more examples:
Width: - wide - narrow
Depth: - deep - shallow
You'll notice something interesting about all these examples: in each case, the noun is like the adjective that defines the positive end of the scale - high rather than low, long rather than short, and so on. See if your pupils can see that pattern and put it into words.
But of course sometimes the words try to trick us. Your height is not how high you are - but how tall you are. So "height" doubles up for the tallshort contrast as well as for highlow. And what's the noun for hotcold? Not "heat", as you might think, but...? We all know a lot of detail, and one of the challenges of writing is to reflect the subtleties of our vocabulary.
Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk
Richard Hudson is professor of linguistics at University College, London