Brainteaser: You have two coins: one is fair (50% chance of heads, 50% chance of tails), and the other is double-headed (100% chance of heads). You randomly pick one coin and flip it. It comes up heads. What is the probability that you picked the fair coin?
Answer: The probability is 1/3 that you picked the fair coin and 2/3 that you picked the double-headed coin.
