Last names
Sometimes you may find it strange that you know a lot about someone and have spent years hanging out with them, but you don’t even know their last name. Do not feel weird about it, that information isn’t really super important, it’s just nice enough that you two get along so well.
Then when you finally come around to ask that person what their last name is and get an answer, you feel like you have finally found that missing piece of the puzzle. Now you know everything about them and have completely the mystery. Congratulations, you are now officially best friends forever.