From William Brubant, this was posted on the groaners listserv. The author is not known.

John and Mary were walking along the shore one Sunday afternoon when they spotted a dock projecting into the harbor. They decided to walk to the end of the dock and sit down to rest . Mary, in her infinite boredom, suggested to John, “While we walk to the end of the dock, why don’t you count the number of slats used to build it, … Read the rest...