Here's a surprisingly subtle and rich logic puzzle, based on one that you can find in various places:
Imagine a tribe of people who are legendary logicians, but who have a curious religious commitment that if they are ever able to determine the color of their own eyes, they must commit ritual suicide in front of the whole tribe at the tribe's daily meeting. The tribe lives on an island with virtually no contact with the outside world, they have no mirrors or reflecting surfaces (for obvious reasons), and they never discuss eye color in any way because they all know what great logical brains they have and so are very hesitant to give away any clues about eye color.
One day, a well-meaning anthropologist visits the tribe, and at their morning meeting, gets up and makes an effort at good relations by saying "Your people and mine are not so different. For example, I see that you have both brown-eyed and blue-eyed people in your tribe, just like I do in my own family". A gasp goes out in the crowd, and the expressions of these logical thinkers rapidly becomes as dark as a grave. The curse of the tribe's commitment is explained to the observer, and he says, "sorry, I didn't realize, but you can all see that what I said is true. So why is everyone acting so crestfallen?"
Let's say the tribe has 20 members, and 5 have blue eyes. The puzzle has two parts:
1) what is going to happen to this tribe that is so awful?
2) what information did the visitor give that the tribe did not already know?