A riddle recently went around work that I later learned was a 'Knights and Knaves' logic puzzle since it involves knights, who exclusively tell the truth, and knaves, who are chronic liars. Below is the first (and easier) one.
You are traveling and come to a fork in the road. One path takes you to your destination and the other to your death. Unfortunately, you do not know which is which. Luckily, two people are there to guide you, a knight and a knave. You do not know who is who, but you are allowed to approach one of them and ask a single yes-no question. What question do you ask to reveal the correct path?
Solution (yes, spoiler alert):
Approach one of the men (it does not matter which). Point toward one of the paths and ask “Would the other man say that this path that I am pointing to is the correct one?" This question works because both men will give the same response. How?
When asking the question, let’s assume you are pointing to the correct path.
The Knight's Logic: You are pointing toward the correct path. You have asked if the other man, the Knave, would tell you that this is the correct path. Well, he only lies, so NO, he will lie and tell you it is the incorrect one.
The Knave's Logic: You are pointing toward the correct path. You have asked if the other man, the Knight, would tell you that this is the correct path. In fact, he would, since he tells the truth, so I must lie and tell you NO.
This shows that if you are pointing to the correct path, both will respond NO.
By the same logic, if you are pointing toward the incorrect path, both will respond YES.
In this way, a single yes-no question will give you sufficient information to reach your destination.
This was prep for the second riddle, which is harder!