For ignition to occur, the concentrated light would need to produce enough heating at a particular spot, and the material at that spot would need to be capable of reaching an ignition or damaging temperature.
That’s why the position of the bottle matters.
The surrounding materials matter, too.
And so does the intensity and angle of the sunlight.
So rather than thinking:
“Water bottles cause car fires.”
A more accurate takeaway is:
“A clear, water-filled bottle can under some circumstances concentrate sunlight enough to create a localized heating hazard.”
