Perpetual motion would be a wonderful thing, if only it were possible - being able to set some machine going and then it would power itself and just carry on & on without end. Free energy from nothing! Which is, of course, why it is impossible - but this wasn't provable until relatively recently. Discussing the search for, and disproof of, perpetual motion on In Our Time were Ruth Gregory (Durham University), Frank Close (University of Oxford) and Steven Bramwell (University College London).
Physics is one of those subjects where I can very clearly see the boundaries of my understanding - as soon as we get to quantum physics or Einstein's theories of relativity I can follow the surface level explanations & analogies, but I'm always aware I don't understand it on a deeper level. I assume the same is actually true of all subjects at some point - I'm not a genius, and I spread my self-education widely among many subjects rather than deeply delving into one - but for physics I can see the fence. It's a peculiar sensation.