That's a very good idea. I would go further and tell the candidate that I don't know the solution myself, thus hopefully putting him/her at ease and making it c

That's a very good idea. I would go further and tell the candidate that I don't know the solution myself, thus hopefully putting him/her at ease and making it clear that I am more interested in the thought and design process, rather than figuring out the "trick" in the question. The tough part here is to choose a good question.