Teaching basic electrics to the young ladies and gents of the RAF, we come to Lenz's Law. There was always one student a bit asleep and bored, but the lesson was followed by a demonstration involving a 1 volt power supply and a nice big copper coil with an iron core fashioned into a horse shoe electromagnet with an iron keeper. When the power was applied, it took quit a bit of force to get the keeper off the magnet. While the students were demonstrating their strength, I would take the previously bored one to one side and suggest it would be a jolly jape for them to remove the crocodile clips from the supply while their mate was pulling on the magnet. If the asked about a shock, I would point out it was only a 1 volt supply, theatrically lick two fingers and put them across the terminals. Matey, being thus assured would wait for my signal and then pull off the clips. Tthis produced a rapidly collapsing field and a decent back emf. The resultant belt would give me a chance to reinforce the learning of Lenz's Law and also drive home the point that apparently innocent things can still bite.
Then one day, out of curiosity, I scoped the back emf, and decided not to do it any more.