John, Mary and the Pier (Groaner)

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, and I'll count the number of slits between the slats?''

John replied, "O.K., I will count the slats, and you will count the slits.''

So the couple merrily trooped down the dock. John counted, "One slat!''

Mary counted, "One slit!''

"Two slats!''

"Two slits!''

And, well, you know how the natural numbers work. Eventually John and Mary approached the end of the dock.

"327 slats!''

"327 slits!''

"328 slats!''

They had reached the end of the dock. Mary was puzzled. "John, there are no more slits. What does it mean?''

John turned to Mary and said, "When you're out of slits, you're out of pier."