I'll tell you what not to do. At one of my DO interviews, I was asked the common question, "how would you explain DNA to a five year old?"
Interviewers: How would you explain DNA to a five year old?
Me: Five years old? Well, I guess they may be a little too young for the "when two people love eachother and stand really close together, they can make a baby" speech, so I guess the transfer of DNA from parent to offspring is out the window.
Interviewers: (Unamused stare)
Me: Uh, well I guess I would take pictures of his relatives, like father and grandfather and explain that the resons that people in the family look alike is that they have a special compound in their body that is passed on. I guess this would be a problem if they were adoped, or didn't really look like thier parents. Uh...
This trainwreck continued on for about a minute. I did well with everything else, but that got a little awkward. Avoid anything outside of the realm of "safe" humor. Somehow they let me in.