I'm not sure there's anything that I understand, at least not fully. There are things I know a certain amount about, but probably nothing that I understand as fully as I'd like.
Also, understanding can be fleeting if you don't think about a certain topic all the time. Take relativity for example: I recall a very specific moment when I was talking to a friend who is actually formally educated in Physics, and we were talking about the speed of light, time dilation, mass expansion, etc. And I had a definite "aha" moment when I understood why mass has to increase with velocity and why time dilation had to occur. But... could I explain it to somebody else now (that was probably 10 years ago), and convey the same understanding? Do I even understand it as well now as I did in that moment? Arguably not. :-(
OTOH, if you just ask "What's a topic that you know a lot about?" I'd say, generally speaking, "firefighting" and "computer programming, especially in Java". shrug
I was tought special relativity and just "accepted" it because experiments confirm it. During a prolonged period of idleness I sat down and derived all formulas from the underlying Lorenz transformation (basically the pythagorean theorm). I think that helped me understand special relativity.
Also, understanding can be fleeting if you don't think about a certain topic all the time. Take relativity for example: I recall a very specific moment when I was talking to a friend who is actually formally educated in Physics, and we were talking about the speed of light, time dilation, mass expansion, etc. And I had a definite "aha" moment when I understood why mass has to increase with velocity and why time dilation had to occur. But... could I explain it to somebody else now (that was probably 10 years ago), and convey the same understanding? Do I even understand it as well now as I did in that moment? Arguably not. :-(
OTOH, if you just ask "What's a topic that you know a lot about?" I'd say, generally speaking, "firefighting" and "computer programming, especially in Java". shrug