I read the first chapter of A Brief History of Time when Dad was still alive, and I got increadibly heavy boots about how relatively insignificant life is, and how compared to the universe and compared to time, it didn't even matter if I existed at all.  When Dad was tucking me in that night and we were talking about the book, I asked if he could think of a solution to that problem.  "Which problem?"  "The problem of how relatively insignificant we are."  He said, "Well, what would happen if a plane dropped you in the middle of the Sahara Desert and you picked up a single grain of sand with tweezers and moved it one millimeter?"  I said, "I'd probably die of dehydration."  He said, "I just mean right then, when you moved that single grain of sand.  What would that mean?"  I said, "I dunno, what?"  He said, "Think about it."  I thought about it.  "I guess I would have moved one grain of sand."  "Which would mean?"  "Which would mean I moved a grain of sand?"  "Which would mean you changed the Sahara."  "So?"  "So?  So the Sahara is a vast desert.  And it has existed for millions of years.  And you changed it!"  "That's true!"  I said, sitting up.  "I changed the Sahara!"  "Which means?"  he said.  "What?  Tell me."  "Well I'm not talking about painting the Mona Lisa  or curing cancer.  I'm just talking about moving that one grain of sand one millimeter."  "Yeah?"  "If you hadn't done it, human history would have been one way..."  "Uh-huh?"  "But you did do it, so...?"  I stood on the bed, pointing one of my fingers at the fake stars, and screamed: "I changed the course of human history!"  "That's right."  "I changed the universe!"  "You did." "I'm God!"  "You're an atheist."  "I don't exist!"  I feel back onto the bed, into his arms, and we cracked up together.