When I was on my first internship, I started developing an Android app, while my friend developed a C# program that read a .txt with info and references from a mail service (in my country it's CTT).

The damn .txt files got really really big, na she had to display all of the data in a listbox (it was a PoC) and when he pressed the item, it had to fill some fields at the left of the listbox.

Needless to say, he didn't learn of multi-threading yet, and I had, so I taught him how to multithread so the app wouldn't lock up while loading the massive .txt file.

The listbox filling made a cool animation (like CMD executing commands from a bat file) and we even implemented a progressbar.

I felt like a badass Dev after that.