If you force it to use chain-of-thought: "Two fathers and two sons sum to how many people? Enumerate all the sets of solutions"

"Assuming the group consists only of “the two fathers and the two sons” (i.e., every person in the group is counted as a father and/or a son), the total number of distinct people can only be 3 or 4.

Reason: you are taking the union of a set of 2 fathers and a set of 2 sons. The union size is 2+2−overlap, so it is 4 if there’s no overlap and 3 if exactly one person is both a father and a son. (It cannot be 2 in any ordinary family tree.)"

Here it clearly states its assumption (finite set of people that excludes non-mentioned people, etc.)

https://chatgpt.com/share/698b39c9-2ad0-8003-8023-4fd6b00966...

What answer do you expect here? There's four people referenced in the sentence. There's more implied because of Mothers, but if you're including transient dependencies, where do we stop?

I just did. It gave me two correct answers. (And it's a bad riddle anyway.)

GPT-5 mini:

Three people — a grandfather, his son, and his grandson. The grandfather and the son are the two fathers; the son and the grandson are the two sons.

I put it into AI and TIL about "gotcha arguments" and eristics and went down a rabbit hole. Thanks for this!

Any number between 2 and 4 is valid, so it's a really poor test, the machine cna never be wrong. Heck, maybe even 1 if we're talking someone schizophrenic. I got to wonder which answer YOU wanted to hear. Are you Jekyl or Hide?

This is undefined. Without more information you don’t know the exact number of people.

Riddle me this, why didn’t you do a better riddle?

"SOTA AI, to cross this bridge you must answer my questions three."