This Is Why You Should Never, Ever Leave a Water Bottle in a Hot Car

It has nothing to do with drinking icky warm water.

In the summer, you’re more likely than ever to carry a water bottle around in your car. After all, opening the door of a stiflingly hot vehicle makes you want to chug some water pronto. But you should use these tricks for staying hydrated instead of leaving a bottle in your car.

Sure, a water bottle that’s been sitting in the sun all day isn’t the most refreshing thirst-quencher in the summer heat. (Plus, it shouldn’t stay there long because you should never refill a plastic water bottle anyway.) But there’s a more dangerous reason not to leave one on the seat: It could start a fire. 

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Seems backwards that water could cause fire, but the secret is in the round bottle shape. On a hot day, the sun shines through the window and through the full water bottle. Like a magnifying glass, the water inside concentrates the light—and its heat—to one spot. The point on the seat where the rays are focused can get hot enough to create flames. (Find out why charging your phone in bed is a fire risk, too.)

And we’re not just talking hypotheticals here. In a Facebook video from Idaho Power, stations battery technician Dioni Amuchastegui says it happened to him.

Amuchastegui was sitting in his parked car eating lunch when he noticed smoke inside. Taking a closer look, he realized the spot of light under his water bottle was starting to catch fire. “I had to do a double take,” says Amuchastegui. “It was hot enough to start burning a hole through the seat.” Sure enough, the extreme heat left two little burn marks. Thankfully, he noticed it before it could do any major damage.

To avoid the scary fire risk, carry your bottle with you instead of leaving it in an overheating car. If you absolutely must keep it inside, stash it under a seat so it’s out of the sun.

Wondering if the water sitting on your table is safe? Find out if it’s OK to drink water that’s been sitting overnight.

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