Riddles, Paradoxes, and Such

Sorry that I haven't posted in a week...I didn't have access to the Internet for a few days :/
Anyway, although I failed to mention this before, I love riddles and paradoxes. Oh, and by riddles, I mean logic riddles, not those riddles that go like "what happens once in a minute, twice in a moment, and never in a thousand years" (which is the letter m, by the way). I thought I'd share a few of my favorites...for the riddles, I'll post the answers on Saturday, so you can guess in the comments if you like haha.


The sentence below is true.
The sentence above is false.
(which sentence is true, which is false?)

What is the probability of getting this question correct?
a) 25%  b)50%  c)75%  d)25%

Monty Hall Riddle
There are three doors. Behind one is money, and the other two doors have nothing behind them. You want to pick the door with money, quite obviously. You choose one door. Of the other two doors, a man, Monty Hall, opens one empty door. You are then offered the choice of staying with your original door, or switching to the other door. Should you stay, switch, or does it not matter either way?

Pop Quiz Riddle
A teacher tells his class, "I will be giving a pop quiz some day next week, and I guarantee you will not know the quiz is happening until you come into class that day."
       Gina, one of the students in the class, thinks about the teacher's statement and determines that the quiz could not possibly be on Friday, because if it were, then all of the students would know this on Thursday night, which would would contradict what the teacher said about the quiz being a surprise.
        She then determines the quiz could not be on Thursday either, because if it was, all the students would know by Wednesday night (since the quiz hadn't yet happened, and thus must be on Thursday or Friday, but since we already determined it couldn't be on Friday, it must be Thursday). Again, this wouldn't be a surprise, contradiciting what the teacher said, and so the quiz couldn't be on Thursday.
Using the same logic, she then determines that the quiz could not be on Wednesday, or Tuesday, or Monday. Thinking that the teacher must have been contradicting himself, Gina decides that no matter what day the quiz is, she will always know.
        Much to Gina's surprise, the teacher announces on Tuesday that he is giving a pop quiz. Gina is surprised, just as the teacher said she would be.
What was wrong with Gina's reasoning?

Liar and a Truth-teller (easiest version)
You are walking down a path when you come to a fork in the road. One road leads to the Truth Village where everyone tells the truth. One road leads to the Lie Village where everyone lies. You want to go to the Truth Village, but don't know which road to take. (oh, and for some odd reason if you take the wrong path you can't turn back LOL)
In front of the doors are two twin brothers, one from each village. One of the brothers always lies, and the other always tells the truth. You don't know which brother is the liar and which is the truth-teller.
You are allowed to ask one single question to one of the brothers (not both) to figure out which door to open.
What question should you ask?

Liar and a Truth-teller (slightly harder version)You are walking down a path when you come to a fork in the road. One road leads to the Truth Village where everyone tells the truth. One road leads to the Lie Village where everyone lies. You want to go to the Truth Village, but don't know which road to take. (again, you can't turn back if you choose the wrong road...)
In front of the doors are two twin brothers that know which village is which, but do not actually come from the villages themselves. One of the brothers always lies, and the other always tells the truth. You don't know which brother is the liar and which is the truth-teller.
You are allowed to ask one single question to one of the brothers (not both) to figure out which door to open.
What question should you ask?

Liar or a Truth-teller (hardest version)
You're walking down a path and come to two doors. One of the doors leads to a life of prosperity and happiness, and the other door leads to a life of misery and sorrow. You don't know which door is which.
In front of the door is ONE man. You know that this man either always lies, or always tells the truth, but you don't know which. The man knows which door is which.
You are allowed to ask the man ONE yes-or-no question to figure out which door to go through. To make things more difficult, the man is very self-centered, so you are only allowed to ask him a question about what he thinks or knows; your question cannot involve what any other person or object (real or hypothetical) might say.
What question should you ask to ensure you go through the good door?

Three people check into a hotel room. The bill is $30 so they each pay $10. After they go to the room, the hotel's cashier realizes that the bill should have only been $25. So he gives $5 to the bellhop and tells him to return the money to the guests. The bellhop notices that $5 can't be split evenly between the three guests, so he keeps $2 for himself and then gives the other $3 to the guests.
Now the guests, with their dollars back, have each paid $9 for a total of $27. And the bellhop has pocketed $2. So there is $27 + $2 = $29 accounted for. But the guests originally paid $30. What happened to the other dollar?

Jenny was 17 the day before yesterday. Next year she will turn 20. How is this possible?



...and there are more, but I won't post them all at once.

--Audrey