A Delicate Conversation

I spoke too soon in my last post when I discussed the lessons my daughters learned from the death of our cat, Elwood. The very next day I had an emotional conversation with one of my daughters... a conversation which is still bothering me.
It all started with a trip to the zoo. During our adventure, we saw "Foxes on Stilts." (I can't remember what they were really called). Near their habitat, a zoo volunteer had a table set up with a fox pelt. There were too many big kids around, so we didn't venture over there. Still, my daughters caught a glimpse of it and wanted to know why "the bones weren't in the fox." I tried to explain to them what a pelt is. They still wanted to know why the bones weren't in the fox. I explained to them that the fox had died and the zoo workers wanted visitors to be able to see the fur up close. The conversation was over. Until....
Later at home, the girls started talking about the fox again and pondering how he might have died. I tried to end the conversation by saying "He was probably very old." Then it happened. "Oh," said Sue Sue. "So animals die when they get old. But not people, right?" I explained to her that even people die when they are old. "But not mommies,right?" she asked. In an instant I knew I had reached a crucial moment. I could lie to her, which I wasn't willing to do, or I could tell her the truth. My answer was something along the lines of this: "Yes, even mommies die, but most of the time it happens when mommies are very, very old. I plan to be around for a long, long time." One of her sisters distracted me and we moved on. Then I happened to glance at Sue Sue. Tears were welling in her eyes. I asked her what was wrong. "I'll miss you when you die!" She wailed. This, of course, made tears stream down my face, too. I pulled her to my lap, hugged her tight and had a quiet conversation with her. I tried to explain that even though Mommy will die some day, it's not going to be any time soon, if I have anything to say about it. We spent a lot of time snuggling that day.
Since then, all four girls have had more questions about death, especially when it comes to Mommy and Daddy. I've tried to be as honest as possible, while still putting it on a level a four-year-old can understand... but really, who can understand death?