Far and away, the most rewarding part of being a scientist or engineer is solving a problem that hasn't been solved before. Seeing a solution prove out and knowing it will be part of a product or service that will further solve problems for other people and make them happier, even if a little bit, and even if they're not even aware they're using it, is the best. One of my earliest inventions is used to allow computer networks to sort of "police" themselves and recover from faults. Knowing that it's being used by literally billions of people everyday even if they're not aware of it is something that makes me smile. I would expect that anyone in any capacity to solve a problem for someone else, be it in technology or medicine or medical technology or law or carpentry or plumbing or just making it easier for someone to get into the church parking lot, gets a thrill from that. We're social creatures and we thrive on acceptance and love, and contributing to someone else's happiness feeds that so well. I think it's the reason we care for our children so much -- to make them happy and safe and improve their lives. It's the way we're built, I think.