As a young teacher I thought I had delivered the perfect lesson on scale drawing. I set the appropriate exercise and expected the pupils to enthusiastically tackle the tasks, applying their new found knowledge. What could go wrong? The folly of youth. Immediately a hand went up asking for help. 'What's the problem?' I enquired. 'You can't do this one' said the indignant pupil. The question involved working out the actual length a real car given the length of a toy car and the scale to which it was built. 'Why not?' I asked rather bemused. 'They haven't even told you what make of car it is. how can you possibly do it?' So much for the perfect lesson.