  Consider the following game: You are given two indistinguishable envelopes,
each containing money. One contains twice as much as the other. You may pick
one envelope and keep the money it contains. Having chosen an envelope, you are
given the chance to switch envelopes. Should you switch?
  The intuitive answer is that it makes no difference, since you are equally
likely to have picked the envelope with the higher or the lower amount.
However, a naive expected value calculation implies you gain by switching,
since you have $ 50\% $ chance of doubling and $ 50\% $ chance of halving your
current winnings, and so if the first chosen envelope contains X, then
switching gives an expected final value of $ (X/2 + 2X)/2 > X $. That seems
like a paradox.
  We prove that the former is the correct answer, and show how the apparent
"paradox" can be resolved.
