You are in a room. There is a table, and three cups on the
table. The leftmost cup contains two white balls, the middle one
contains one white ball and one black ball, and the rightmost cup
contains two black balls.
You leave the room for a while, and someone exchanges the places
of all cups. You can't look into the cups but you can pick one ball up
from a cup and look at it, then put it back. How many times will you
have to take a ball from a cup and put it back, in order to know which
cup is which? You do have to put the ball back before taking
another one.
Published in Njet Problem! collection 1996-05-08.