When I substitute teach, I always have backup plans — books and topics we can talk about if we run out of material. One of my favorites used to be The Book of Totally Useless Information by Don Voorhees (MJF Books, 1993). It contains an entry on obscene gestures entitled “When did it first become offensive to extend the middle finger?”
Now, such information can be very useful for travelers. For example, this entry also explains how the “V for Victory” sign can mean something totally different (“shove it”) in England and Australia when reversed (palm in), and other pitfalls. So on one occasion two years ago I taught a lesson on obscene gestures at the end of a period.
Of course the kids loved it. But another teacher heard about it and reported me. So the next time I was in that unit of the high school, I had to explain to the unit principal what I had done. He was understanding, but I had to sign a memo saying I’d used poor judgment, wouldn’t do it again, and if I did, wouldn’t be allowed to teach there any more.
BTW, the book claims the middle-finger gesture’s origin is unclear, but it goes back to at least Roman times, although I’ve seen other explanations.