Why are you crying, Mommy?

Today I was sitting there reading an article online about fathers & miscarriage and something in it hit a chord with me, and the tears started flowing (yet again...). My 6 yr old stopped in his tracks and asked me why I was crying.  I told him I read something that made me sad, and left it at that.  It's hard to explain to kids why you're sad. While I don't try to hide my grief from them, I don't want them to worry, either.  I'm not always sad... Promise! Most of the time when they ask questions, we're able to talk openly about what happened to the baby without any problem.  But other times I can't even think about him/it without tearing up!  Sometimes the hurt is buried beneath the hustle and bustle of daily life, sometimes it subsides enough to push it aside and smile and laugh with my kids and friends, but sometimes, no matter how hard I try, the grief  lurks just below the surface waiting for something, ANYthing to trigger the waterworks.  How can you explain to a child that grief comes and goes, but doesn't really disappear?  I would say that one day they'll understand, but as a mom, my hope would be that they never do!