For over 4 years, I used C#'s in and out keywords without fully knowing how they work, and any articles I read didn't seem to help me understand, for some reason. Now, this very specific SO answer made the whole thing fully make sense in my head. It's probably the giraffes.

Obviously co/contravariance are way more than just the in/out keywords, but for some reason that specific way of dealing with the concept never seemed to make total sense to me. Now that I understand, looking at the official docs again makes me wonder how this ever confused me.

Sometimes it takes me literal years to understand something to the point where I feel like I could actually talk about how it works, but during that time, I can still use and understand it on, like, a different level

(though I guess this is also part of why rubber duck debugging is so effective: if you explain something to someone or something else, you have to think about it in a different way than if you're just dealing with it on your own)