In the 1920s, chemists searching for a solution to engine knock helped create one of the most widespread public health disasters in history. Leaded gasoline promised smoother-running cars and bigger profits, but at a cost that would not be fully understood for decades. In this episode of The Mistakes That Made Us, Stephanie Einstein Tashiro traces the rise of tetraethyl lead from a celebrated technological breakthrough to a global environmental contaminant. Along the way, she explores the scientists who warned of the dangers, the children whose lives were affected by lead exposure, the battle to remove lead from gasoline, and the legacy that still lingers in soil near roads and cities today. It is a story of innovation, unintended consequences, and how one decision can shape generations.