The most valuable thing I learnt in my chemistry lessons wasn’t how to put my finger through a Bunsen burner flame without it hurting. Nor was it that hair really is as flammable as they say it is.

It was actually a little piece of grammatical advice that has stayed with me ever since – not least because it was so curious: “Stop the crocodile”.

What my chemistry teacher was referring to was a sure-fire way of eliminating one of the more common mistakes people make in writing: using ‘s’ when they should be using ‘c’.

The word in question was practice (or practise). And the difference is not one of UK versus US English – although, certainly our American cousins do things differently in this case.

Instead, the logic is simple:

• Any time you are using the verb, you use ‘s’ – like you do when you say ‘stop’. So, I practise my grammar with exercises like this.
  
• If you are using the noun, it’s a ‘c’, as in crocodile. Grammar practice is important for schoolchildren – pity it’s no longer on the curriculum.

In fact, there is a whole host of terms that ‘stop the crocodile’ applies to. For instance:

License and Licence: He may have been licensed to kill but James Bond was still booked for speeding without his licence last week.

Practise and Practice: Dr Jones liked to practise his juggling at his practice in Cheltenham.

Advise and Advice: You can advise people as much as you like but you can’t get them to listen to your advice.

Devise and device: The Greeks devised a plan using a cunning device that we call the Trojan Horse.

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