Or for those of you who don't want to go through the bother of being well-entertained by an excellent story:
There are 3 possible outcomes:
Donkey 1, Donkey 2, or Tickets
Let's say you pick one of the curtains, you then have a 1/3 chance of having chosen either the Donkey 1, Donkey 2, or...
Everyone who believes there is only a 1/2 chance needs to read (or preferably listen to the audiobook of):
The Curious Incident of the Dog in the Night-Time
Not only is it an excellent book, it references this paradox
(yes, it IS a paradox)
And CLEARLY explains why it is favorable to...