“I bought a bike lock a few months ago,” I announced. “Not here, somewhere else. Anyway, it rusted shut yesterday. Could you sell me a lock that won’t do that?”

The bike store man blinked. “I don’t know any lock that won’t do that. Things rust. If you don’t want something to rust, you have to oil it.”

“Oh…….” I was properly chastised. “Erm. What kind of oil?”

“Any lubricant for moving parts,” he explained, slowly, for all the stupid people in the room.

“Do you, ah, sell oil here?”

“Yes.” He pointed to a large white plastic bottle. “It’s £2.”

“Well, I’ll have one of those.”

He took the bottle off the shelf and my £2 coin to the cash register. I hovered around uselessly.

“Would you like me to open it for you?” he offered kindly.

“Ah, thanks.”

He snipped off the lid and handed it over to me. “Careful,” he cautioned. “The bottle is open now.”

Such kindness to the stupid children of Cambridge University.