Firefighters are warning drivers not to leave clear bottles in water in their cars because with the right set of circumstances it can cause a fire. Sunlight gets magnified through the clear bottle then reflects a focused beam on to the seat of the car. According to the Oklahoma Midwest City Fire Department that beam can get as hot as 250 degrees. Although chances of this happening are low. It has happened and you can never be too safe. If you don't believe me watch the video from Idaho Power below.
Source: Country Living and You Tube