Well, I have to teach it to my fifth graders, so it kinda jumped out at me.

"But still, that solution is based on "trial and error",
Based on the responses from the "assistant", it's how he came to the answer as well. Hence the need for more information to eliminate other possible solutions.
"Of course one could argue you could build an algorithm to find such answer,"
Not with the amount of inferencing and lateral thinking involved in deducing the answer, not to mention the fact that prime numbers occur at random intervals and cannot be predicted. You are welcome to try, though.

Okay, I just remembered a riddle I read a long time ago. I hope I remember it correctly.
A man is in a square room with 4 windows, one on each wall. All of the windows face south. He sees a bear go past one of the windows.
What color is the bear?
Good luck.
