I have cola-bottle (1.5L) that is half-filled with water. The other half was air, of course. I put it in the fridge (~ -14 C) and after about an hour, before i could see ice formation but when it was very cold nonetheless, i saw that the bottle hull was deformed to the inside, the same way if you smash it (the Dutch for it is a "deuk", don't know how to translate that, "indentation"?). So, the pressure inside has become smaller than the pressure outside. According to the gas law, the pressure should drop as the temperature goes down and the volume remains constant. But the temperature of the air around it was -14 degrees Celsius, so that pressure would have been smaller also. Can anyone explain this "indentation" ?