Leaving a clear water bottle in your car may seem completely harmless, but under very specific conditions, it can create an unusual fire hazard. A transparent, curved bottle can refract sunlight and potentially focus it onto a small area of the vehicle’s interior. The concern isn’t that the water itself burns, but that concentrated sunlight can heat a surface enough to cause damage or, in rare circumstances, ignite combustible material.

For this to happen, several factors generally need to line up. The bottle must allow sunlight to pass through it, strong direct sunlight must reach the bottle, and the light must be concentrated onto a suitable surface. Dark upholstery, paper, plastic, or other heat-sensitive materials may absorb the concentrated energy. However, leaving a water bottle in a car does not mean a fire will automatically occur.

The simplest precaution is to keep bottles away from direct sunlight. Take them with you when leaving the vehicle or store them somewhere shaded and secure. The same principle can apply to clear glass containers because glass can also transmit and refract sunlight. The material of the container isn’t the only consideration—the shape, position, and intensity of the sunlight matter as well.

Extreme heat inside a parked vehicle is another reason to avoid leaving water bottles behind for long periods. High temperatures can affect plastic containers and the quality of their contents. It’s also wise to remove other heat-sensitive items, including medications, electronics, batteries, and aerosol products, whenever practical.

The risk from a water bottle is unusual and highly dependent on circumstances, but avoiding it is easy. Before leaving your car, check that clear bottles aren’t sitting in direct sunlight, especially on seats or near the dashboard. A few seconds of precaution can eliminate a potential hazard while also helping protect the items inside your vehicle.