actually, the glass on a coca cola bottle is pretty thick. There was this picture in a physics book I saw once where they'd encased a bottle in plastic (they were demonstrating refraction). Something like a full third of the width was glass.
While encasing a bottle to check this might be out of the question for you, you could probably do a displacement test of a bottle to check the disparity in volumes (outside versus inside).
Anyways....
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