Brain Teaser 14: Four Card Problem

A nice little logic problem I found here at curiouser.co.uk.

You are presented with the four cards represented in the illustration below.  Each card has a number on one side and a letter on the other.

You are told that every card that has a vowel on one side has an even number on its opposite side.
Which card or cards must you turn over in order to determine whether or not that statement is true?

Answer