"Right children, we're going to talk about dinosaurs. Can anyone define 'dinosaur'? ... Well, here's the official definition. Write it down and learn it for tomorrow."
Or maybe you'd prefer to start with some pictures, an extract from a film or television programme, or a story?
Why should grammar be so different? Why not just settle for a few clear examples, then get straight to business? For example, here's one route into prepositions. Pick a preposition - any preposition. Let's suppose you chose of. Now select a page or two from your favourite book and read it to the class - but tell them to shout out whenever they hear of. If by chance you happened to select chapter 1 of Harry Potter and the Philosopher's Stone, one paragraph on the second page yields eight examples including these:
the corner of the street
the first sign of something peculiar
a trick of the light
he thought of nothing
a large order of drills
Write up the example on the board, but spread each one across three columns, with of in the middle column. (We've missed out a couple of tricky examples and we suggest you do the same. With grammar, be selective and concentrate only on clear and helpful cases.)
Now get the class to mess around with the examples. Help them to see that column 3 contains nouns or noun phrases while column 1 contains mostly nouns - corner, sign, trick, order - but also examples of the verb think (thought, thinking).
What does of do in these sentences? It allows a following noun to affect the meaning of a preceding noun or verb. In other words, it's a kind of grammatical glue (or double-sided sticky tape) that holds together words on either side of it.
Now you're ready for terminology. Tell pupils that of is a preposition, and ask them to think of other prepositions. By the end of the lesson they should be quite good at spotting prepositions, and may even think about them more when writing. And although you haven't given a definition, what does that matter? You've done something more important by triggering an interest, and a deeper understanding, of one small but important aspect of language.
Richard Hudson is professor of linguistics at University College, London Geoff Barton is headteacher of King Edward VI School, Bury St Edmunds, Suffolk