A water bottle can start a fire in your car. Really?

I was in the parking lot at work and someone was talking to me through the car window and asked "Do you keep the water bottle in your car"?  I thought that was a weird question.  

Turns out the sun blazing through the car window, then in turn through a clear bottle, then the water acts like a magnifying glass.  I have never heard of this happening to someone.  Have you?