Dallas Clayton noticed a sign (top) and decided to add an appropriate companion piece (bottom) in honor of author Shel Silverstein. If Clayton's name is familiar to you, it's because he's an accomplished children's author and illustrator himself, and Tiffany interviewed him last year.
Link -via Dude Craft
He was a very prolific author/catroonist/poet especially well known for children's books. Died in 1999.
His first collection of poems (and one of his most famous books) was "Where the Sidewalk Ends" -- hence, the appropriateness of the post script signage.