“Why are you so tired, daddy?”
Well, Mr. Moony, I was working all day today, and the work is still not finished, and I have so much more work to do still.
“Are you gonna work while I sleep?”
Probably.
“Why do old people die, daddy?”
Well... because they've been alive for so long, their body's been working so long, and after so many years of their body working, everything gets real tired; their heart gets tired, their chest gets tired, their tummies get tired, their legs get tired, and their eyes get tired, and their heads too. Everything gets so tired to the point where it's ready to say, All done, bye now.
Laughter, and then: “Are we gonna die soon?”, asked with a smile and a trail of laughter still.
Hopefully not you, Moony, because you're still very young.
“But you're gonna die soon.” The voice of conclusion, not a question.
Sooner than most, I think, but hopefully not too soon.
He then nodded in silence and after a beat or two changed the subject before drifting off to sleep.