It is a small and very clean paradox: a coin, a toss and new information are enough for intuition to begin to distribute the probabilities incorrectly.
The two-sided coin
Riddle statement
In a bag there are two coins:
- one coin has heads on both sides;
- the other is a normal coin, with heads and tails.
You take one out at random, throw it and it comes up heads.
What is the probability that the other side of that same coin is also heads?