The WeWatt bike is an energy station that allows passers-by to charge their mobile device by physical activity. A WeWatt bike is most often used individually, even when multiple people use it simultaneously. To solve this problem, we design the WeWattTree, an installation that uses human-powered energy to filter the environmental air. By orchestrating subtle motion gestures, we aim that the WeWattTree can nudge people to join in, and encourage them to socially interact with one another while biking.