Not one thing, three things
Adams had a post years ago where he said that anyone, if they put their mind to it, can become among the top 20% in the world at something. If you can become the top 20% at two things that complement each other, that's a good recipe for success.
As for himself, he figures he's a pretty good cartoonist, he's pretty funny, and he understands business pretty well. Put the three together and he's rich.
