Remember in school where we had the experiment of using a magnifying glass with the sunlight going through it onto a sheet of paper and the the paper would burn? It seems a water bottle can do the same thing.

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Firefighter David Richardson of the Midwest Fire Department in Oklahoma talked with KFOR said,

The sunlight will come through, when it’s filled with liquid, and act as a magnifying glass as you would with regular optics. It uses the liquid and the clear material to develop a focused beam and sure enough, it can actually cause a fire, a combustion.

Idaho Power workers saw this first hand.

They posted a video of what the sun can do through a water bottle after a small burn was discovered on the seat of one of their vehicles. In the video, they hold a bottle over the seat and it begins to smoke and reaches a temperature of 213.

It's even possible that the hand sanitizer we've no grown accustomed to carrying everywhere could be a fire threat, too, if left in direct sunlight in your car. The alcohol used in certain hand sanitizers could release a vapor that could ignite is heated up.

Basically, your best bet is to bring your hand sanitizer inside with you and cover your water bottles to protect them from direct sunlight.