JMcAndrew asks, “I’d think at age 5 Dennis would already know what a zoo is. Did Henry take Dennis to some kind of strip club and tell him it was the zoo?”
From the expression on Henry’s face, I think that might be the case, though this seems odd and inappropriate for a DtM strip!
I’m reminded of the alleged bar or strip club named The Library, a great place to spend an afternoon “studying”…
Comics I Don't Understand 
Can confirm that there are bars named The Library in multiple college towns I’ve lived in.
As for it being a strip club, I’d think that the women there do SOMETHING. Dance, at least.
based on the art work I’d guess this was a really old one. first thought it was a parody strip, but the signature looks like Ketcham, so maybe not, unless whoever did it faked that too.
Yes, I think this is a very early strip. It took a few months for the strip to settle in, and for Henry to stop taking Dennis to places like strip clubs. But Dennis saying more than he should has always been happening. Usually in front of guests. “I don’t see no blue streak when she talks.” “I want to see you drink like a fish!” “Roast beef! I sure am glad to see that! Dad said we were going to have a couple of stuffed shirts for dinner!”
Women coming out and doing nothing suggests something other than stripping was happening in other rooms.
This is a real one and can be found in the 51-52 complete reprint book. Early on, Ketcham would toss in slightly edgy gags better suited to his magazine clients.
Serendipity and Baader-meinhof, I just read about the ancient Celsus Library in Ephesus that had a secret underground tunnel linking it to the public brothel; quips the author, “It gave ‘lust for knowledge’ a whole new meaning.”
Don’t forget the classic “Dennis the Menace” cartoon where Dennis looks at his mother and says, “I see your little, petrified skull… labeled and resting on a shelf somewhere.”
It’s actually partly real. Here’s a link that explains it:
https://screenrant.com/far-sides-gary-larson-dennis-the-menace-captions/
I never saw Henry as particularly smart, but how much of an idjit would he have to be to take a loudmouth kid like Dennis to a strip club?
Dennis The Menace had a lot more edge when it was new. My personal favorite (which I’ve only seen in collections, so I don’t know the date) has Henry pulling a shotgun out of the closet (which I’m guessing was a “going hunting” thing), with Dennis asking if “It’s a burglar, can I have his flashlight?”.
Similar can be said of a lot of the legacy strips – Family Circus got fairly wild at times, Blondie wasn’t always just about Hungry Hungry Dagwood, etc – that have been eroded over decades into the bland nothingness we’re familiar with today.