First rule you need to teach, question everything!Even if homeschooling is not an option, adults can do so much for young one's minds to develop, soo easily, with so little effort .
Bragging here, forgive me, I just love my daughter.
I kept feeding my now 20 year old daughter books . I simply spoke with her about topics/issues I am interested in, about things that concern me, personally, globally, in terms of health, economics, politics, you name it . Essentially form very early on, adapting the way I speak and what we talk about to her capabilites as good as I could. This in turn sparked her curiousity about many things not covered at school, or at least not from the background or perspective I do look at things . I started challenging her thoughts and believes I think when she was 7, by way of dialogue and information rather than argument and discussion . And, I am keeping the promise I gave myself, and her, even before she was born : I am never lying to her .
2 weeks ago, we talked a bit about QuantumMechanics ( the few bits of which understand enough of the words that I can actually talk about it ), and I managed to get the dialogue to one of the dilemmas of the many worlds theory . I described the Problem using an admittedly rather clumsy metaphore ( I told her it was not a really good one...). She came up with the solution, out of nothing, just like that ! She was like "But what if...?" . And I smiled and said "Yeah, and that is pretty much how it very likely really is ."
She simply solved the Problem .
A few days later, I gave her the copy of Kahneman's "Thinking. Fast and Slow" I had bought to give away . Will do her english a lot good, and we will have a lot more things to talk about in the future.