She's only just turned 6, and has a pretty sharp mind, but we were watching Gravity before bed and she asked why there is day and night.
I used a couple of objects to explain how the Earth revolves around the sun, and then we got into how seasons work and such. Sounds silly but it was a really fun evening. The kid has a real interest in science and how things work, and I felt really good being able to explain this to her.
Before she went to bed we made a decision to read something interesting about the world, space, the oceans, whatever we feel each night so she can go to sleep thinking about it. A proud dad moment for me, and I'm smiling while I write this
Edit: even better, I have another one I can teach this sort of thing to once she's older. Happy days!!!!