Educating Children

Sitting on a bench in the swimming pool changing rooms, 5.45pm, yesterday. My son is getting dressed.

Me: Sorry, what did you say? I was a million miles away.
My son: No you're not mummy, you're here!
Me: It's an expression. It means my mind is a million miles away (gesticulating to the ceiling), it's not in my head (pointing back at it), it's far away (hand sweeps the distance)
My son: I bet it's 12 steps for me to climb from your feet to your head! Let me try. One! (he steps on my foot) Two! (clutching my shoulders with his hands his foot is on my shin. He continues to clamber up me but I've stopped paying attention) Nine!
Me: Aargh! What are you doing? (as he lands a foot on my shoulder)
My son: Ten! Ten steps to your head mummy! (As his sole touches my crown)
Me: Ha ha.
He tumbles down
Me: There's another expression that means the same thing.
My son: What's that mummy?
Me: "The lights are on but no-one's home"
My son: I don't understand mummy.
Me: The lights are on, I'm here, awake but no-one's home (pointing at my head) - my mind's far away.
My son: Oh