When my second grader was awake a half-hour after bedtime on a school night I was tired and annoyed. When he was still up 1.5 hours later, I was starting to despair. I went into his room and - once again -- scratched his back and sang him a lullaby. Aiden interrupted my version of "Baa Baa Black Sheep" to ask, "Dad, have you ever seen an atom?"
Seeing that Aiden was still quite awake, I decided to answer his question. We discussed atoms, protons/neutrons/electrons, and smaller particles. Having scaled down, Aiden wanted to scale up: can you combine atoms with other atoms? I explained what a molecule was, using water as a simple and tangible example. My son surprised me by telling me how many water molecules are in a raindrop, something he'd learned at school. From there we worked out how many atoms are in a raindrop. The chain was suddenly complete and the physical universe snapped into place in his eight year old brain.
The joy was contagious: suddenly there was no way we could stop.
From atoms we moved on to the elements. Aiden made an intuitive leap to hypothesize about the melting points of metals versus other elements. Melting points led to freezing points led to what would happen if you stuck your hand into liquid oxygen. Aiden quickly applied his new knowledge to wonder to which specific nano-scale the cold would shatter his fingers.
I thought things were maybe getting a little silly when Aiden switched to the cloning of woolly mammoths, but I was surprised when he used the term "arctic species." Then I was floored when he went on to discuss the ethical consequences of cloning one animal versus reintroducing the whole species. Having decided he was in favor of cloning -- the species that is -- he ended the conversation by asking what "mammoth" meant as a word, without the modifier "woolly."