Life, liberty, and the pursuit of happiness. Friends, Romans, countrymen. Blood, sweat, and tears. Government of the people, by the people, for the people. Stop, look, and listen. The good, the bad, and the ugly. Sex, drugs, and rock and roll. These phrases do not just happen to have three elements. Three is the magic number that human brains find most satisfying, most memorable, and most persuasive. Two feels incomplete. Four feels like too much. Three is the sweet spot where information becomes sticky. This is not cultural conditioning or arbitrary convention. The preference for triads appears across languages, cultures,...