I went to math camp as a kid in middle school. John Conway was one of the lecturers. He was clearly a brilliant man, slightly eccentric in the way that most great mathematicians are. But he was humble enough to devote several days of his time to hangout with a bunch of nerdy kids and teach us about ordinals, knot theory, and I believe Ramsey numbers. Really nice of him to do that for what I'm sure was pennies if it paid at all.