Turns out leaving water bottles in your car can be dangerous. A man in Boise, Idaho learned this the hard way, when a plastic water bottle in the front seat of his truck started to smoke. Under the right (or wrong) conditions, the water filled plastic bottle can act like a magnifying glass and catch the sunlight, concentrating the energy on one point. In this particular case, it caused the car seats vinyl to burn.
While this is not likely to happen, it is possible… so if you are going to keep water bottles in your car, it’s best to keep them out of the sun!