Hans and Gretchen were walking along the shore one Sunday afternoon when they spotted a dock projecting into the harbor. They decide to walk to the end of the dock and sit down to rest (chat, have a smoke or whatever).
Gretchen, in her infinite boredom, suggests to Hans, “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?”
Hans replies, “Ja, sehr gut, I will count the slats, and you will count the slits.”
So the couple merrily troops down the dock. Hans counts, “One slat!”
Gretchen counts, “One slit!”
And, well, you know how the natural numbers work. Eventually Hans and Gretchen approach the end of the dock.
They reach the end of the dock. Gretchen is puzzled.
“Hans! There are no more slits! What does it mean?”
Hans turns to Gretchen and says, “When you’re out of slits, you’re out of pier!”