Who knew? As most of the country deals with a heatwave, firefighters are reminding people to stay hydrated.

But when you do, don’t leave half-full water bottles in the car; they could start fires. “If you do leave a bottle of water in your vehicle and the sun catches it just right, it could act like a magnifying glass,” says Charlottesville firefighter Frank Scopelliti. “[It could] heat a specific spot and start a smoldering fire that could eventually become a real fire.”

Acknowledging that the idea might sound a little far-fetched, the Midwest City Fire Department in Oklahoma conducted an experiment, holding a water bottle in front of a piece of paper on a sunny day. Sure enough, the concentrated sunlight quickly burned a hole right through the paper.

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