Pocket Change

You and a friend pass by a standard coin operated vending machine and you decide to get a candy bar. The price is US$0.95 but after checking your pockets you only have a dollar (US$1) and the machine only takes coins.

You turn to your friend and have this conversation.

You: Hey, do you have change for a dollar?
Friend: Let's see... I have 6 US coins but, although they add up to US$1.15, I can't break a dollar.
You: Huh? Can you make change for half a dollar?
Friend: No.
You: How about a quarter?
Friend: Nope. And before you ask I can't make change for a dime or nickel either.
You: Really? And these 6 coins are all US government coins currently in production?
Friend: Yes.
You: Well can you just put your coins into the vending machine and buy me the candy bar, and I'll pay you back?
Friend: Sorry, I'd like to but I can't with the coins I have.

What coins does your friend have?

Missing $1

My friend Sony told me this riddle back in Middle School:

So three monks are traveling home when it starts to rain. Seeing this, they stop by the nearest hotel and check in. The room rate is $30 for one night for all three of them so the monks split the cost three-way and pay $10 each.
The manager later finds out that the three who checked into the hotel are monks and decides to give them a discount. The manager takes $5 and tells the bellboy to go give the monks back some of their money. The bellboy decides to pocket $2 of the $5 and gives $1 to the each of the monks.

So here's the problem:
Technically the monks all paid $9 each for their room. The bellboy pocketed $2 which totals up to $29. What happened to the last dollar?

Programming: Swapping Variables

Here's a pretty simple programming problem:

Let A equal some integer and B equal some different integer. How can you swap these variables without using a temporary variable?

The Monty Hall Problem

I read this in the book

The Curious Incident of the Dog in the Night-Time

Suppose you're on a game show, and you're given the choice of three doors: Behind one door is a car; behind the others, goats. You pick a door, say No. 1, and the host, who knows what's behind the doors, opens another door, say No. 3, which has a goat. He then says to you, "Do you want to pick door No. 2?" Is it to your advantage to switch your choice?

Children

One of my co-interns gave me this riddle a few years back:

So there's a family with two kids.
One of the kids is a girl. What is the probability that the other kid is girl?
(It's not 1/2)

Measuring

An easy riddle Robert reminded me of during a midnight-ish snack at Yechon.

So you have two buckets, one measures 3 liters and one measures 5 liters. How can you use these two buckets to measure exactly 4 liters of water?
(You have infinite water supply)