The Scratches
This puzzler today was sent in years ago by a listener in California.
Here it is.
"This tale starts on Halloween night in the town of Soda Springs near Donner Summit in the Sierra Nevada. As we got in the car to go to a Halloween party, the windshield was coated with a fairly thick layer of ice. We could not find an ice scraper, so a friend of ours grabbed an empty aluminum soda can and then used the unpainted bottom to scrape off the ice.
So, he is holding the can as you would drink it, except he is using it to get the ice off the windshield, rubbing it across the surface a bunch of times.
So then, off we went to the Halloween party without a worry about the windshield because we knew that aluminum is softer than glass. We thought that the only thing that could scratch glass was a diamond. I think we have all heard that before.
So the next afternoon we go to the car and to our dismay we find a bunch of deep scratches in the windshield.
When we got home we looked up the hardness values for aluminum in the CRC Handbook for Chemistry and Physics. So we looked it up and sure enough, aluminum is considerably softer than glass on the Mohs Hardness Scale. So it stands to reason that no scratches should have appeared on the windshield."
And the puzzler is, why were there scratches on the windshield?