A minor act of arson
The lesson was going very smoothly.
Students were getting on with their work and I was moving around, offering help.
Then I heard a yelp. Two boys had leapt to their feet and were desperately jumping up and down. They were putting out a fire.
I was surprised by my calmness. Without raising my voice, I asked the two students to leave the classroom. Speaking to them outside it turned out that they had set fire to some string with a lighter. They admitted that the prank had been more explosive than they had expected. They accepted the inevitable trip to the headteacher's office with good grace.
Two teenage boys enjoying a mini-rebellion. I'd obviously rather they'd have chosen someone else's class.