I'm pretty sure this is an old stand-up routine from one comedian or another, but I can't put my finger on it. Any idea, folks?
Wikipedia: To cope with the difficulty of hobo life, hobos developed a system of symbols, or a code. Hobos would write this code with chalk or coal to provide directions, information, and warnings to other hobos. Some signs included "turn right here", "beware of hostile railroad police", "dangerous dog", "food available here", and so on.