Here is a puzzle which I feel is, at least tangentially, related to the envelope puzzle.
I have two coins: one fair and one mangled. I flip them so that they both land on a table behind a curtain where you cannot see them.
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[1] What is the probability that they have both landed on the same face?
[2] How would your answer change if I partially pulled the curtain back to reveal the mangled coin?
[3] How would your answer change if I revealed the fair coin?
[4] Are your answers to the previous questions "facts" or "opinions"?[/list]