Two Kinds of Knowledge
Sirius Black did not know a lot. He knew the best way to make a woman fall all over him, he knew how many stars made up most of the constellations and he knew the easiest way to sneak out of school. But it was all trivial. There was so much important stuff that he didn't know. He didn't know who the Muggle prime minister was, he didn't know what the conversion rate was from galleons to pounds, and he didn't know how to buy a train ticket. But there was one thing that was more important that didn't know. He didn't know how to tell his best friend that he was in love with him.
Remus Lupin knew many things. He knew how to banish a boggart, he knew how to borrow books from a Muggle library and he knew how a bus station worked. This was important knowledge. He didn't know anything that most people seemed to know instinctively. He didn't know how to smile so someone else's heart melted, he didn't know how to mingle at parties, and he struggled to make friends. But there was one more instinctual thing that he didn't know. He didn't know how to tell his best friend that he was in love with him.