It's a rather difficult riddle, based on the riddle two guys where one always tells the truth and the other is lying. In that riddle you can ask them one question and then you have to decide which is the liar and which is the truthteller.
Some of you might remember this riddle from the old forum. Please don't spoil it for everybody right away.
So here goes.
You are the most eligible prince in the kingdom and as such the king has invited you to his castle so that you may choose one of his three daughters to marry. The oldest princess is honest and always tells the truth. The youngest princess is dishonest and always lies. The middle princess is tricky. Sometimes she tells the truth and sometimes she lies. You don't want to get screwed over at this, of course.
Here's the problem: The king wants a smart man to inherit his throne and he doesn't want you to favor any of his daughters because of looks. So he complicated it. You don't know which princess is which. You are allowed to ask ONE of the princesses (any) ONE yes-or-no question ONE time. Then you have to make your pick. What would that question be?
As you will be forever married to one of the princesses, you want to marry either the oldest (truth-teller) or the youngest (liar).
Since it doesn't matter which princess you ask the question as you don't know who is who, you need a question that will universally reveal the information that you need regardless of which princess you ask.
Here is the solution. Don't open it if you don't want to mess up your fun.
The question you need to ask is following.
QuoteIs sister A younger than sister B?
It is assumed that you asked sister C. It doesn't matter who you ask, it is only important that you make that sister answer a question about her other sisters. Your goal is to pick the younger one. That means, when you get a "Yes", you pick A, otherwise you pick B.
This is a possibility of all combinations. Sister 1, 2 and 3 are the youngest, middle and oldest sisters.
Case 1:
You asked the youngest and she said:
YES
That means:
1 = C
2 = B
3 = A
Case 2:
You asked the youngest and she said:
NO
That means:
1 = C
2 = A
3 = B
Case 3:
You asked the oldest and she said:
YES
That means:
1 = A
2 = B
3 = C
Case 4:
You asked the oldest and she said:
NO
That means:
1 = B
2 = A
3 = C
In case you asked the middle sister, it doesn't matter what she says. You're going to pick one of her sisters and that's a win-win situation.