Whats is your definition of common sense?

i wonder what common sense is. Really. How do rate what common sense is if the average american doesn't agree that everything is common. i would consider it common sense to know micheal phelps won 8 gold medals in the olympics but because somebody doesn't know it, does that mean they dont have common sense? And common sense would be to lock your door but i have plenty of friends who dont lock their doors. And i would think common sense would be you could tell me about the last 5 presidents and when they were in office and out of it. But then again i asked some questions about that and you wouldnt believe how many wrong answers i got? So what knowledge is common sense and whats just not common?