Contemplate This

http://market-ticker.org/akcs-www?post=230800

You're a psychotic mass-murderer about to commit your horrific crime.
You walk into a place intending to shoot it up.  Maybe it's a school, maybe it's a shopping mall, who cares.  The important thing is that you're intent on mass-murder and you are armed to the teeth.
As you walk in you survey your intended victims.
And here the story diverges into two scenarios.
In one you note on the way in a sign that says GUN FREE ZONE.  You know this is backed up by a law.  The place might be a school, a playground or some other public place.  You're reasonably sure there are no weapons there -- other than yours, of course.
In the other you see some number of people with pistols on their hips.
Has anything just changed?
You bet it has.