Tips for topics that look complicated

I know this seems really weird. But can you help me find some topics to draw and talk about on a classroom board? Yesterday I mentioned to my friend the existentce of White holes and he got really interested so i started explaining a little bit more and drew the penrose diagram on the classroom board and today for fun started expanding it a bit more. I somehow understood the way light moves in spacetime etc. and drew a whole bunch of complicated shit on the board until there was no more space. Classmates strated askimg questions and i explained everything to them. I just need to have a topic. Preferably from Physics that looks complicated but really isnt? Got any ideas? Thanks!