One of the epiphanies of my programming career was when I grokked function pointers. For a while prior to that I really struggled to even make sense of that idea, but when it clicked it was beautiful. (By analogy I can sort of understand what it might be like not to understand pointers themselves.)

Then I hit on the idea of embedding a function pointer in a data structure, so that I could change the function pointed to depending on some environmental parameters. Usually, of course, the first parameter of that function was the data structure itself...