Water isn’t going to suddenly catch fire because sunlight passes through it.
The concern is the surface receiving concentrated light.
Depending on the circumstances, that could include parts of the vehicle’s interior or an object sitting nearby.
Dark materials generally absorb more sunlight than light-colored surfaces, although whether anything reaches a hazardous temperature depends on the specific circumstances.
And again, this is a potential localized heating issue—not a guarantee that ignition will occur.
What About Plastic Water Bottles?
Plastic bottles are especially common in vehicles, which is why this warning tends to circulate so often.
The concern isn’t that the plastic itself is acting like fuel simply because it is sitting in the car.
Rather, a clear bottle containing water can potentially redirect sunlight.
Glass containers can also transmit and bend light, so the underlying optical principle isn’t exclusive to plastic.
There’s another reason to avoid leaving disposable plastic bottles in a hot vehicle, too: heat can affect the bottle and the quality of its contents, even though that is a separate issue from the sunlight-focusing concern.
Does a Hot Car Make the Fire Risk Worse?
A parked vehicle can become extremely hot in direct sunlight.
