Spoiling the punchline
On Friday I was doing transmission of electrical power with my HL class and set off on my usual example to show how much power would be lost if it was transmitted at a lower voltage. Unfortunately I chose values that gave a power loss of about 1 Watt which didn't quite give the look of surprise I was expecting on the faces of my students. So I reduced the transmission voltage and upped the power delivered to get a more respectable 0.1 MW. The problem now was that the students all new what would happen when I added a transformer to step up the voltage. Sort of ruined the story. They'll probably remember the principle though which is the whole point really.