This was discussed among my friends and the guy who put it forward said this answer by palascosas is the right answer. However we weren't exactly in agreement so that's why I posted it here to get a better feedback. I see that there is not a definitive answer here actually.
There are two absolutely identical bottles placed in outer space, one containing a liquid and the other empty. You cannot see the inside, and are not allowed to move either of them. How do you determine which contains the liquid?