Something that my husband and I agreed on a long time ago, was to answer all of the questions they ask and answer them honestly. Of course, also use discretion in regard to their age when they ask. We don't want to lie, make things up or just shut them down with "You're too young", though. We want to create an environment conducive to open dialogue as they grow older. We want them to feel comfortable coming to us about anything.
Alex walked up to me and said "Mom, what is rape?" I immediately knew he heard it on the news, because I had just heard the news story end. My reply to him was "Give me a minute to think about how I'm going to explain this. You're kind of young for this subject." He responded with "Mom, I'm 9 and a half... I'm only 3 and a half years from being a teenager. I can handle it."
What him asking this REALLY meant was, I had to carefully and age appropriately explain 2 subjects. I did it and it went better than I thought it would. He did say "So you and Dad have had sex then, since you have kids." Then there was a few more questions from him on the sex side of this discussion, before he went off to play video games. Before anyone freaks out... he gets the gist of it, but not the full mechanics of sex. I think that's a good place for him to be right now, at his age.
I'll spare you all the details of the explanation... unless you really, REALLY want to know how I approached the subject.
That was enough excitement for my day.