Leaving a clear plastic water bottle on a car seat might seem harmless, but under the right conditions it can create a surprising fire hazard. When strong sunlight passes through a bottle filled with water, the curved surface can act somewhat like a magnifying lens. The light may become concentrated onto one small area of fabric, vinyl, or another dark surface inside the vehicle.
Fire departments have demonstrated that this concentrated beam can produce enough heat to scorch upholstery. Actually starting a fire is uncommon because the bottle, sunlight, and surface all have to line up correctly for long enough, but the possibility is real. That is why firefighters often recommend keeping bottles out of direct sunlight rather than leaving them on seats or dashboards.
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