As if body changes aren’t enough, the tween years also bring major changes in moods, behaviors, interests, and the way they think. Suddenly, your child can sling some attitude, text without looking, and argue you to death-all at once! Many of these new “skills” are blamed on raging hormones, but the truth is that most of them are actually caused by all the shiny new neurons and hardwiring going on in your adolescent’s brain.

As recently as a few decades ago, scientists thought the brain was finished growing around age two, and “data entry” was all that happened after that. Today, through advanced medical imaging techniques, we know that the brain begins a second, large growth spurt around age twelve and finishes up in the early...