The goal of this clinical trial is to help adolescents and young adults between the ages of 13-25 with Cystic Fibrosis (CF), medically stable, able to speak and read English, and are not experiencing a CF - related exacerbation, who are already active to remain, or gradually encourage them to increase their levels of physical activity Participants will be asked to utilize a smartphone program, called NUDGE that we have developed. NUDGE is a chatbot with evidence-based features known to help teens make progress toward health goal: * Set and review goals * Self-monitor progress * Provide feedback on goal attainment * Revise future goals