If you want to show them how Linux is a legitimate OS don't tell them its free right of the bat.
Tell them it runs on many of the servers they use to access their school's databases, websites, and security.
Here is a fun fact: Many of the planes used by our government have a unix backbone. They might be surprised to learn that what keeps planes flying are systems built on versions of Red Hat/Fedora.
Tell them that most computer science majors must have some background in Linux to graduate. It is an operating system most (probably all) computer science majors had to learn.
Tell them that the NSA (National Security Agency) aka the authority on Network Security for the DoD (Departement of Defense) uses Linux on many of their systems. Tell them all about SE Linux and why the NSA supports it.
Tell them that Linux and Windows are not so different and that you can use many Windows programs on both operating systems. Introduce them to Open Office (you won't believe how many people you will convert with Open Office).
Again don't open with "It is free" the response will almost always be negative.