You are given 4 cards, two face up and two face down; say, 3,8, Red, Brown, and asked which card(s) must be turned over in order to test the truth of the proposition that if a card shows an even number on one face, then its opposite face is Red?

Apparently only a small percentage of those tested give the 'correct' solution - although the 'great majority of subjects agree with the logic of the solution, once it is explained to them'.

The 'correct' solution is given in the article, but it is worth attempting to solve it first, if you are not already familiar with it.

Question: Why is the 'correct' solution in fact questionable?