It's not supposed to be especially tricky. Yes, as soon as you realise that the horn is a bucket with a very deep section that is narrower than the assumed thi

It's not supposed to be especially tricky. Yes, as soon as you realise that the horn is a bucket with a very deep section that is narrower than the assumed thickness of a coat of paint, the surprise dissipates. I believe the point of the exercise is an introduction to curves with infinite length but finite area under them, in order to expand one's intuition about such objects, which is then transferable to other examples like space-filling curves.