Sometimes a single visible piece of information is enough to change the weight of an entire situation. This riddle is deeper than it seems.
The urn that reveals itself
Riddle statement
An urn contains 100 balls. Some are red and the rest are green.
You don't know how many red there are, except this: the number of red balls was chosen at random with the same probability between 0 and 100.
You draw a ball. It comes out red and you put it aside.
Now you are going to draw a second ball from the same urn.
What is the probability that this second ball is also red?