She stepped onto the biggest of global stages to face the most diverse of audiences, and she made it count. "How dare you?" she kept saying to some of the world's most powerful people. "You are failing us," she told them. Sometimes, a moment and a person align. For 16-year-old Greta Thunberg — whether you admire her or dislike her, and there are plenty of passionate partisans in both camps — Monday was that time.