One day I was sitting in the classroom while my students worked their way through some exercises. Because it was getting close to the middle of the day, I suggested to them that once they had finished the exercises, they should probably go straight off for lunch. I reminded them of the time that the class would start again in the afternoon, and left them to it. I've always found this to be a successful tactic because it gives the students an incentive to finish the tasks ahead of them. Their reward is an extra long lunch hour.
And so, in dribs and drabs, off they went for lunch.
By the time the official lunch time arrived, only two students were left. By coincidence, both were of Chinese descent.
At that moment an email arrived in my in tray. Naturally I read it. It contained a joke, and so I laughed out loud.
The two students looked up and for a moment I felt embarrassed. Teachers aren't supposed to do that sort of thing.
And then I realised that both of the students were looking at me with very puzzled expressions on their faces. The penny dropped. I'd been laughing in English, with a Yorkshire accent, and they hadn't understood me.