Try to explain it this way:
A computer is a car
Linux is the motor
GNU is all of the frame, tires, nuts and bolts, the air conditioner, the stuff in the instrument panel etc.
a distro is the car brand (ie. Fedora=Ford)
So therefore non-free OSes are cars that have the hood, doors (except there is no drivers side window so you can climb in like a NASCAR driver), and trunk welded shut. The lugnuts on the wheels are in a design that doesn't fit the tools in the trunk (that you had to break into to get)
and you can extrapolate it from there :)
-------- Original Message --------
Subject: [fsf-community-team] Terminology. GNU/Linux
From: Don Saklad <address@hidden>
Date: Thu, December 10, 2009 10:41 am
To: address@hidden, address@hidden
When using the term GNU/Linux in advocating important principles, Boston City Hall people fail to comprehend the term. Would any of you kind folks have any hints, tips or pointers?...