Saying I Don’t Know

How do you say "I don't know"? More often than I would like I am presented with an opportunity to tell a client or potential client how little I know about topic XYZ. In consulting, each opportunity presents itself as new twists to an old problem or some completely new animal that catches you totally off guard.

If you have had success in your past endeavors you usually have an answer or a process that quickly derives an answer in your back pocket. For those days when you get blind-sided, just how do you say you are clueless without coming off as inept?

1. Discuss something you have done that is similar
2. Demonstrate some degree of familiarity with the problem space (though perhaps not a solution)
3. Admit you simply do not know

I have found that trying to know everything is generally a recipe for disaster. It comes back to bite you in the end. I will usually opt for total transparency whenever possible and point out where I may be lacking in hopes of fostering a trust relationship with the client. This in turn sometimes leads them to grant a little latitude or even an allotment of time to learn something new.

Since I am always looking for new ways to not look totally moronic, how do you say "I don't know" gracefully?