A man has 2 children. He tells you one is a boy born on a tuesday. What is the probability the other is a boy to?
(with the loosest assumptions, ie. we assume a-priori there is a 1/2 chance to give birth to a boy independent of past births, no twins, and no more boys or babies are born on tuesdays than other days of the week (on average)).
Its not very hard, but it show how hard it can be to combine information - quickly, is the probability highter than, lower than or equal to 1/2?