Your toddler has roughly twice as many brain connections as you do. Growing up runs on deletion rather than addition.
In the first years, more than a million new neural connections form every second. By age three the brain holds about double an adult's — a deliberate surplus, laid down before anyone knows which parts will be needed.
Then comes pruning. Connections that get used are strengthened. The rest are thinned away. The brain spends early childhood building far more than it will keep, and the years after that deciding what to keep.
Which changes what nurture means. It works less like pouring something in and more like casting votes on what survives, and the ballot is repetition rather than instruction.
Back-and-forth conversation builds what matters. Babble answered strengthens the circuit. A question taken seriously keeps the curiosity wired.
Every answered babble is a vote, and what you keep answering is what stays.