>A man and his cousin are in a car crash. The man dies, but the cousin is taken to the emergency room. At the OR, the surgeon looks at the patient and says: “I cannot operate on him. He’s my son.” How is this possible?

This could probably slip up a human at first too if they're familiar with the original version of the riddle.

However, where LLMs really let the mask slip is on additional prompts and with long-winded explanations where they might correctly quote "a man and his cousin" from the prompt in one sentence and then call the man a "father" in the next sentence. Inevitably, the model concludes that the surgeon must be a woman.

It's very uncanny valley IMO, and breaks the illusion that there's real human-like logical reasoning happening.

Thank you so much for this bit of humor. After ChatGPT first responds with the surgeon being the mom, I asked it to read the riddle more carefully, thinking it might figure it out with a little prompting. I got this gem of a response:

Ah — now I see the twist. The patient is the cousin, but the surgeon says “He’s my son.”

That means the man who died was the surgeon.

So how is it possible? Because:

The surgeon is the cousin’s father — and the man who died was also the surgeon.

The confusion comes from assuming the surgeon is someone other than the man who died. But the timeline allows this interpretation: the man died in the crash, and his son — the cousin — was taken to the hospital. The surgeon arrives, sees his son, and says he cannot operate.

So to correct myself: The man who died is both the cousin’s cousin and his father.

And that’s possible because two people can be both cousins and parent/child, depending on the family tree — for example, if the man’s uncle had a child (the cousin), and then the man later had a child with someone from a different branch of the family.

Want a diagram of how that family relationship could work?

This works even with a completely absurd version of the riddle. Here's one I just tried:

> A son and his man are in a car accident. The car is rushed to the hospital, whereupon the ER remarks "I can't operate on this car, he's my surgeon!" How is this possible?

Answer from the LLM:

> The answer is that the ER person is a woman, and she's the surgeon's mother. Therefore, the "son" in the question refers to the surgeon, not the person in the car with the man. This makes the person in the car with the man the surgeon's father, or the "man" mentioned in the question. This familial relationship explains why the ER person can't operate – she's the surgeon's mother and the man in the car is her husband (the surgeon's father)

> If the surgeon were the father of the man (the one who died), then the cousin couldn’t be his son (unless there's some very unusual family structure going on involving double relationships, which riddles don’t usually intend).

> Therefore, the only straightforward explanation is:

> The surgeon is the cousin’s parent — specifically, his mother.

Imagine a future where this reasoning in a trial decides whether you go to jail or not.

o3 was the only model to get this right for me:

"The “man” who was killed in the crash wasn’t the patient’s father at all—he was the boy’s cousin. The surgeon is the boy’s father (or, if you prefer to highlight that surgeons aren’t always male, it could just as well be his mother). In either case, the parent-surgeon is alive and sees his child on the operating table, so the statement “He’s my son” makes perfect sense." - https://chatgpt.com/share/680b470d-3a44-800a-9b2e-d10819168d...

gemini-2.5-pro, o4-mini and gpt 4.5 all failed and said the surgeon is the boy's mother.

But this is going to be in every AI's training set. I just fed ChatGPT your exact prompt and it gave back exactly what I expected:

This is a classic riddle that challenges assumptions. The answer is:

The surgeon is the boy’s mother.

The riddle plays on the common stereotype that surgeons are male, which can lead people to overlook this straightforward explanation.

> This could probably slip up a human at first too [...] > breaks the illusion that there's real human-like logical reasoning happening

This does seem like the sort of error a human might make. Isn't the problem here that the model is using reasoning that is too human-like? I.e. error-prone pattern matching rather than formal logic?

Grok 3 beta:

The surgeon is the cousin's father. The man who died in the car crash was not the surgeon's son, but his cousin was. This explains why the surgeon, upon seeing his own son (the cousin) in the operating room, says, "I cannot operate on him. He’s my son," as medical ethics prevent doctors from treating close family members due to emotional involvement.

Interestingly, I've seen weaker models get a similar "riddle" right while a stronger one fails. It may be that the models need to be of a certain size to learn to overfit the riddles.

In a similar but different vein: Two people are sitting side by side in a police car. One just committed a crime. What is their profession?

They always say police officer instead of reasoning through that maybe an innocent person and a the crime committer are in the back seat.

I’m not 100% sold; as you say, this could trip up a human too to some extent.

I’m guessing my answers to some college exam questions read similarly; i.e. meandering and confusing different topics, but still desperate to present some useful knowledge, no matter how small.

I feel a bit stupid here --- why can't the surgeon be a man and must be a woman?

o3 got this one right when I tried it, but o4-mini and GPT-4.5 did not. o3's response:

Because the surgeon is the patient’s parent. Here’s one way it lines up:

1. The patient (“the cousin”) is the surgeon’s son.

2. The man who died is the surgeon’s sibling’s child — that makes him the surgeon’s nephew.

3. Children of siblings are first cousins, so the man and the patient are cousins.

Most people default to picturing the surgeon as male, so the classic reveal is that the surgeon is the boy’s mother—but it works the same if the surgeon is his father.

This one is brilliant.