Grim Adventures Of Billy And Mandy (the) Easter Egg - Where the Sidewalk Ends!!

In the episode Tricicle of Terror, Sperg wants revenge on Billy's tricicle, so he grabs it. He walks over to a ledge and says, "This is where the sidewalk ends!" Where the Sidewalk Ends is a book by Shel Silverstein. You can also see a dog hanging on the ledge, which is the same dog on the cover of the book!