Back on my blog in 2008, I said that if I ever taught statistics, I'd write the following on the blackboard the first lecture:
Colin comes up to Barry and Frank and says, "hey guys! I have a coin here which is biased but I won't tell you which way or by how much. What's the probability of the next toss being tails?". Barry replied, "given what we know, it is 0.5". Frank looked astonished. "How can you say 0.5? Given the coin is biased, that's the one probability we know it can't be." Who is right, Barry or Frank?