This is why you should never leave a water bottle in your car.

The reason is that water bottles, usually spherical in shape and made of transparent plastic, can act like magnifying lenses. When filled with water, they are capable of focusing sunlight into a single point. At midday, the temperature inside a car can reach nearly 60°C. With sunlight passing through the bottle and converging on a spot, the heat can rise even higher, creating a fire hazard.
Another study by the American Academy of Pediatrics found that as time passes, the temperature difference inside cars remains minimal whether the windows are completely closed or slightly open. In both cases, the temperature inside can increase by about 40°C within one hour, even if the outside temperature is only 22°C.

The takeaway: It’s best to break this habit before it’s too late. If you must leave bottled water in the car, at least wrap it carefully and avoid placing it in areas directly exposed to sunlight.