Two-envelope problem

[Tuan V Nguyen <t.nguyen@garvan.unsw.edu.au>]

Hello folks,

        I have the following problem which may be interested you:

        There are two envelopes containing money. One 
envelope contains twice as much as the other, however since the 
envelopes look identical you cannot tell one apart from the 
other. You choose one envelope and you see that it contains 
$100. Is it better to keep the $100 or choose the other 
envelope?

        Any idea ?

        Tuan