Swing the arm up to replace the wiper blade. Remove the wiper blade. Accidently hit the wiper arm w/ no blade on it and the spring tension snaps it down on the windshield. Nothing but spring loaded metal wiper arm against the windshield. Ouch!
It's pretty self-critiquing...
After that you'll learn to put a big thick folded towel down against the windshield before you change the blades.
Ask me how I know...
