I recently came across “one of the oldest bar jokes in the world” and decided to try to explicate it.

From Wikipedia: One of the earliest examples of bar jokes is Sumerian (c. 4500–1900 BC), and it features a dog: “A dog walked into a tavern and said, ‘I can’t see a thing. I’ll open this one’.” [1] The humor of it is probably related to the Sumer way of life and has been lost, but the words remain.

Others have speculated that the punchline relies on a pun or the understanding that the tavern was a brothel or that the thing that gets opened is a door, but I don’t think any of these is the answer.

I suspect the original joke was more like a riddle: “A dog walked into a tavern, but he couldn’t see a thing. Why?” … “Because his eyes were closed!” It’s a silly joke suitable for children that still sort of works today. And, like the chicken crossing the road, once everyone knows the joke, you longer have to give the full context.

And I think the context may have been this: that in that culture, closing your eyes was a common way of indicating (when telling a story) that the character couldn’t see because it was dark or because he was blind. So doing that while telling the joke and then opening one eye flips the scenario, making it literal–and silly–that the dog’s eyes were closed.