Levchin: We hit it off really quickly. I have this IQ bias—anybody really smart, I will figure out a way to deal with.

It was very positive. Both of us are really competitive and really—not mistrusting, but not willing to assume that the other guy knows what he's talking about. When we met, we sort of hung out socially, and then one night we had this showdown where we sat around in this café for like 8 hours and traded puzzles to see who could solve puzzles faster—just this nonstop mental beating on each other. I think after that we realized that we each couldn't be total idiots since we could solve puzzles pretty quickly.