And I'm left with a dilemma. How do I answer her without completely freaking her out?
For children, the feeling that adults can provide them with security is all-important: they need to believe that you are in control of everything outside their understanding. Their zone of safety starts and ends with the adults in whom they invest their faith.
So to say, for example, "I don't know why we die; no-one knows. Some people believe there's life after death; some people don't. Yes, mummy will die one day," isn't exactly the recipe for a good night's sleep.
But, equally, I'm not prepared to lie outright - to fill her mind with beliefs I'm not sure about, or to open her mind to possibilities that are too much for even an adult mind.
So where do I draw the line?
In the end, I usually tell her that I don't know why we die, but not to be scared, because when she's an adult she will understand and won't be afraid (even though that's not necessarily true). I tell her that the numbers never end because we can always add one more... then sit pondering on the nature of eternity whilst my daughter - happy with the answers - snores, oblivious.
I remember having unshakable faith that adults knew what they were doing; understood what it was all about. And it was my first day as a teacher that shattered the illusion. My class were sitting, looking at me as I spoke, and I suddenly thought - "you actually think I'm an adult, don't you?" "You think I know the answers, and I'm just as clueless, scared and unsure as you probably are."
It's scary to think that - in many ways - we're all 18 and stumbling around in the dark. The more I talk to others, the more I realise the deep insecurities that hide behind our masks of feigned certainty.
Which is pretty scary when you think about it.
But perhaps, in this uncertain world, admitting you're "not sure" is the best way. To exude confidence, to declare you have all the answers, is surely simply to advertise your ignorance. Because no-one does. No, not even Donald Trump, whatever he may say. Which is what makes the prospect of his presiding over the USA so freakishly scary.
It is not uncertainty perhaps that we need to fear; but certainty.
Unless you're six years old it's bedtime.
In which case, it's best just to assume that mummy knows best, shut your eyes and clock up a few zzzs...