How do the lungs develop after taking their first breaths outside the womb? What cellular events and changes early in life give rise to lung malfunction and disease? To help answer these questions, scientists have constructed the first single-cell atlas of postnatal lung development in humans and mice. The research could help provide a more detailed understanding -- at the level of individual cells -- of which genetic and epigenetic factors affect lung health across the human lifespan, starting from birth.