As a general rule, I try to keep my nose out of other people's business. It can actually be a lot of fun. I was once visiting a friend at Harvard, and I crossed paths with a tour group, with a guide explaining the history of the statue (of John Adams?) and the student tradition of rubbing his foot for good luck. I quietly listened in. After the guide finished and started leading the group off to their next destiantion. She threw an aside at me, "So, did I get it right?" She had assumed I was a Harvard student, I guess. All I said was, "sounds good to me."

Probably my favorite example of keeping my mouth shut happened ~12 years ago when my dad and I were test driving new cars. We were driving a Nissan, I think, and the salescritter was talking about how aerodynamic the car was, tested by NASA windtunnels, etc. At the time, I was a summer intern at NASA Ames, working in the same building as all the high-performance numerical fluid flow people. My dad is a computer architect, and those NASA people are big customers of his work. Dad and I just looked at each other and looked right back at the road without saying a word. (NASA does airplanes, not cars. And, if they did do a car, for whatever reason, it would be an American car, not a Japanese one.) Sure, it would have been fun to nuke the salescritter, but it was more fun to see what else he was making up.

Of course, if somebody asks you for advice, then it's another matter entirely...