The Self-Referential Kangaroo
31 July 2026
I remember a joke that someone told me a long time ago. I suspect it was while I was an undergraduate at Rensselaer Polytechnic Institute, in the 1970s.
“Have you heard the one about the self-referential kangaroo?”
“No.”
A kangaroo walks into a bar.
The bartender says, “We don’t serve kangaroos here.”
The kangaroo says, “Wait! Let me tell you the self-referential kangaroo joke.”
So the kangaroo tells the joke. It finishes, waits for a reaction, and the bartender says, “I don’t get it.”
“That’s it,” says the kangaroo. “That’s the joke.”
People usually react with puzzlement. “I don’t get it either.”
Some people, especially if they are mathematically inclined, have that moment of “I don’t get it.” And then a magical moment occurs. They see that they have become the bartender.
And then, for an instant, they see an infinite hall of mirrors. That is funny. To me, it is also unexpectedly beautiful.
The joke has no punchline.
The recursion is the punchline.
Once you have seen recursion, you begin to recognize it everywhere. Computer scientists build programs that call themselves. Mathematicians construct proofs that refer to smaller versions of themselves.
For example,
To find all of Alice’s descendants
- List Alice’s children.
- For each child, find all of that child’s descendants.
This joke makes me laugh because I see the infinite mirrors suddenly appear in my own mind. But it’s even more delightful when I watch that same realization subtly ripple across the face of a friend as I tell the joke. For a moment, we both experience something very close to awe.
End Note
I did not invent this joke. Someone told it to me, and it has frequently re-surfaced in my memory.
Recalling it again today, I wondered if it has been discussed on the internet. Searching for published versions before writing this essay, I found many recursive jokes and many kangaroo bar jokes, but not this one.
If you know an earlier source, I would be delighted to hear from you.