One of my favorite things to do when I am teaching is explaining the whole Linux thing to my undergrad students. It takes a while to understand that no, their instructor isn't crazy (about this), there really is a free operating system out there that's pretty much running the Internet, supercomputers, and the DVR back in their dorm room. So what is it about Linux that's made this operating so popular for everyone from individual users to enterprises to governments? Is it the cute penguin? Linus Torvalds' charming personality? Here are five attributes that epitomize Linux as a successful operating system.