Firefighters and safety experts are warning drivers about an unusual hazard that can happen when clear, water-filled bottles are left inside parked cars. Under the right conditions, sunlight passing through a curved bottle can become concentrated onto a small area of the interior. ☀️🔥 That doesn’t mean every bottle left in a car will start a fire—the risk depends on the bottle’s shape, position, sunlight, and what the focused light hits.

The science is similar to using a magnifying glass to concentrate sunlight. A transparent plastic or glass bottle filled with water can bend and redirect light, potentially focusing it onto dark upholstery, paper, plastic, or another heat-sensitive surface. The bottle itself doesn’t need to become dangerously hot. Instead, the concern is the localized heat created by concentrated sunlight.

A parked car can already become extremely hot, especially on a bright summer day, making it a poor place for certain objects. Clear bottles aren’t the only concern, either—glass containers and other transparent or reflective objects can interact with strong sunlight. There are also separate reasons to avoid storing drinking water in a hot vehicle for long periods, particularly when temperatures become extreme. 🥵

The easiest solution is also the simplest: move the bottle out of direct sunlight. Put it in a shaded compartment, or better yet, take it with you when you leave the car. Don’t panic if you’ve forgotten a bottle before—this is an uncommon situation, not an automatic fire hazard. But if you see a clear bottle sitting in strong sunlight, taking a few seconds to move it is a tiny precaution that costs virtually nothing.