‘That’s because it’s chuffed,’ said Colon with a sudden flash of inspiration. ‘See, you’ve heard of “chuffed”. That’s where it comes from.’

Nobby looked at his friend in admiration. ‘Is that why, sarge? I never thought of that!’

‘And when it’s had enough of a chuffing, there’s enough magnetism to hold the train on the iron road, see?’

The last phrase was delivered in a rush in the hope that no more questions would be forthcoming. But it doesn’t work like that with children. The elder boy had had enough and decided to show off the knowledge gleaned from friends who had been there earlier in the day. ‘Isn’t it to do with reciprocating motions?’ he said, with a glint in his eye.

‘Ah, well, yes,’ blustered Colon helplessly. ‘You’ve got to have your recip-roca-tory motions to get the right kind of chuff. And when everything is chuffing and reciprocatoring, away we go.’

The smaller child was still puzzled, as well he might be. ‘I still don’t understand, mister.’

‘Well, perhaps you’re too young to know,’ said Colon, taking refuge in the excuse used by exasperated adults through the millennia. ‘Very technical stuff, your chuffing. Probably shouldn’t even be trying to explain it to children.’

‘I don’t think I understand either,’ said the mother.

‘You know clockwork?’ said Nobby, coming to the rescue again. ‘It pretty much goes like clockwork, only bigger and faster.’

‘How’s it wound up?’ asked the boy.

‘Ah yes,’ said Colon, ‘that chuffing noise, of course, is the winding up. And when it’s wound up, then off it chuffing goes.’

The smaller boy held up a clockwork engine and said, ‘He’s right, Mum, you wind them up and away they go.’

Bemused, the lady said, ‘Right … well, thank you, gentlemen, for a comprehensive little talk. I’m sure the boys were fascinated.’ And she handed Colon several coins.

Colon and Nobby watched the happy family as they climbed on to the cart behind Iron Girder. And Nobby said, ‘It’s a nice feeling, isn’t it, sarge? Being helpful to people.’

Moist’s cab halted at the palace, and he helped an exhausted Drumknott up the stairs. Amazingly he was beginning to feel sorry for the little chap, who was looking like a lotus eater who had run out of lotuses.[23]