Firefighters have watched this unfold in real vehicles, not just in controlled demonstrations. A half-finished water bottle, left on a seat in direct sunlight, can act like a magnifying glass. The curved plastic and water bend and focus the sun’s rays onto a single point, heating the fabric until it begins to smolder. One Oklahoma firefighter described a car seat burned straight through by a focused beam of light, the owner stunned that a simple bottle could do so much damage.

It’s an invisible risk because it hides in plain sight, tucked into cup holders and rolling under seats. The fix is painfully simple: don’t leave clear plastic bottles in a hot car, especially in summer or in direct sun. Take them with you, stash them in the trunk, or store them out of the light. One tiny habit change could be the difference between a safe ride home and a car filled with flames.