Mark Dominus in The Universe of Discourse:
There are two boxes on a table, one red and one green. One contains a treasure. The red box is labelled “exactly one of the labels is true”. The green box is labelled “the treasure is in this box.”
Can you figure out which box contains the treasure?
It’s not too late to try to solve this before reading on. If you want, you can submit your answer here:
A) The treasure is in the red box
B) The treasure is in the green box
C) There is not enough information to determine the answer
D) Something else
More here, including the solution to the puzzle.