When dd was 3.5, my grandmother died after a lengthy illness. While we did not live nearby, she and dd had spent several holidays together over the years, and with an upcoming memorial service, I knew I had to explain to her what had happened.
I sat dd down before bedtime and gave her the talk about great-grandma going to heaven, etc. etc. She looked at me with great intensity through the whole speech.
When I finished, I asked her, "Do you have any questions for Mommy?"
"Yes," DD replied, seriously.
"Go ahead," I said, "You can ask me anything."
"Mommy," she said thoughtfully, "Why do some plants grow in water?"
They're always full of questions. Just sometimes not the ones you are expecting.