Take a potato, peel it and boil it. It’s still a potato. Now mash it, and observe the difference. It’s no longer a potato - it’s just potato. It’s changed from being a thing - notice the indefinite determiner a: a potato, a thing - to being stuff. Technically, the countable noun potato has changed into a mass noun.

English takes the difference between countable and mass nouns very seriously. Countable nouns can be counted - or, more accurately, the things they refer to can be counted - so we can have one potato, two potatoes, three potatoes, four ... But in spite of the old counting rhyme, we can’t count potato. So countable nouns have a plural but mass nouns don’t.

Another important difference is in the determiner - in fact, if the noun is singular that’s the only way we know whether it’s countable or mass. Think of the effects of a, every and many compared with some, much or no determiner at all: countable: I ate a potatoI ate every potato I didn’t eat many potatoes.

mass: I ate some potatoI ate potatoI didn’t eat much potato.

It’s not just potatoes that are treated like this. The whole point of eating is to transform something which starts off as some kind of individual outside us into a substance inside us. At some point in this transformation, the grammar flips from countable to mass - a sheep (countable) changes to mutton (mass), a tomato (countable) changes into soup (mass), a grain of wheat (countable) changes into bread (mass).

The same contrast runs right through the noun system and allows us to present alternative views of the world. Take The TES that you’re reading at the moment. Is it a paper, or just paper? Answer: both. You could describe it either way, and the choice is yours. It all depends how you see things.

As speaker or writer, you have to decide how the paper is relevant. When you buy your TES it’s a distinct item, this week’s TES, with all the expectations that raises. It is definitely a countable item, as in a paper: I bought a paper. (Equally of course it could be my paper or the paper, but these determiners don’t show the contrast between countable and mass.) But by the time you recycle it, it’s relevant only because of what it’s made of - paper - so it’s just paper. Our use of the noun signals the way the product has changed, losing its original identity.

Grammar is like a sandpit: it gives children a chance to play with the relations between substances and things. Get them to change countable into mass or the other way round, eg, countable dog into mass as in We don’t eat dog, or mass grief into countable as in a grief ago.

This grammatical playfulness is at the heart of much of the writing in our language that we find most thought-provoking and emotional. Grammar isn’t only about rules and relations. It’s also, of course, about how we express our deepest feelings.

Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk