How habits form: why you do things you never decided to do

Today, your thumb opened an app on your phone before you had decided anything. That was a habit at work, run by a part of your brain you never think about. See how habits form from the inside: the cue, the routine and the reward, the basal ganglia, dopamine and its prediction error. Then the harder part: how to start a new habit, how long it really takes (forget the 21 days you may have heard of), and why an old one is so hard to break.