Don't fake it.
Unless you are going for an acting gig, figure that if you try to fake it, there's a real chance that it will read to the interviewer as odd. There's no harm in already knowing the answer to the question, particularly if the interviewer is asking one of the standard riddles.
Listen carefully and answer
Particularly with riddles, there is a real possibility that the question may sound the same except for a change in wording that really does change the answer. Always listen carefully to make sure you do, in fact, know the answer. A lot of interviewing is showing the behaviors that are relevant to being successful in the workplace, and one of them is almost certainly being attentive and a good listener.
But if the answer is quite honestly one that you already have memorized, say the answer. If the interviewer seems surprised - feel free to say you've been doing a lot of interviewing and this one has come up before. It's certainly not your fault you've heard it before, and the interviewer should know that this is a known problem with asking such static questions. Being able to remember the answer to a question you've already been asked before is also a good trait in almost any job!
Bonus points for creativity.
If this is one of the creative thinking type riddles, feel free to give the answer and then have something funny or creative to say about it. Keep it short and sweet, but if you have something interesting and personable you can say, go for it.