envelope paradox?? Maths can sometimes be nearly as interesting as physics... There are two envelopes on a table. Both have cash in, you dont know how much but one has twice as much as the other You can pick just one envelope, and you are allowed at most one swap. You choose envelope A say, and see that has £100 in it. Are you better off on average to swap and choose envelope B??? Envelope B could have £50, or £200 in it - 50% chance of each So expected value if you swap is 0.5*50 + 0.5*200 = £125 So on average you are better off swapping - how can that be????