Turbo pascal help for me! The polynomial example taught me how to use pointers. Before that, I could only use static arrays up to a certain length.

Learning about heap allocation was euphoric. I kept beaming because I had unlocked infinite memory, and people around me didn't get why I was such a happy teenager.

To be fair, I already knew about memory regions from PEEK/POKEing on a commodore as a child, but it was always static and pre-populated.