When my kids ask me I'm just going to put it on the level...
Answer there questions and leve it at that... mostly what I have found to be true is that if you just talk planly with kids then they will understand and make most conections on their own.
So just tell the truth and your kids will respect you for a long time... fib, refuse to tell and they'll dig in one way or another way, often the one way you don't want them to learn by... like getting pregnant, or a V.D... I know that I would reather just give my kids the talk, that way I would not have to hold there hand when thay get sick, pregnant, or like a good frend of mine DIE. My kids will be the future of me and my wife's blood line and will be our harts and souls, I just don't want to risk my kids lives on ignorance.