How many of you keep spare spark plugs in your pocket or backpack or wallet? I don't, but then again the car I make payments on still has a warranty. Oh, and roadside assistance, so I won't need to break a window to get into it.
In a preview for the show Hacking the System, one of the guys tries to smash one of the side windows on a Jeep Grand Cherokee with a mallet. It doesn't work. So the other guy dismantles a spark plug.
Of course, don't smash windows on cars that aren't yours. This was done to prove a point. But if you've locked your keys in your car with a child or pet or melting Ben & Jerry's inside, go get a spark plug.