This puzzle was first posed by John von Neumann, and it is still quite tricky to work out.
You have a coin that is biased so that the probability of a head is not the same as the probability of a tail. The probabilities of each are fixed but you don't know the probabilities.
How can you use this coin to produce an outcome where both alternatives have a 50% probability?
The solution should work irrespective of the degree of bias in the coin, although clearly not if it lands heads 100% of the time!
Answer at 9.00 on Monday