The answer I was thinking of is "a quarter and a nickle". The riddle postulates that one of the coins is not a quarter, and that is demonstrably true; one coin is a nickle and the other coin is a quarter.

Of course, the point here is that the form of the question (calling it a riddle) sets up thought parameters for most people. They won't consider it a trivia quiz, but rather an exercise in flexible thinking. The question is deliberately fuzzy in its language and, of course, that's part of the amusement factor in solving (or hearing) the "answer".