Cameron Ford, Academic Advisor, SFU Chemistry Department answered on November 28, 2009, A:If you found this answer useful, please consider
Indeed the vacuum inside the juice bottle is the key to the explanation of this phenomenon. The lids of vacuum sealed bottles and jars have the ability to flex in or out, depending on the quality of the seal. You can always tell if the seal has been broken on a jar or bottle, if the centre of the metal lid rises up in a dome shape. It should be indented, held in a concave shape by the vacuum inside the bottle. Anyone who has done home canning has had the experience of hearing the tops of jars snap as they bend inwards while cooling. This is caused by a vacuum forming inside the jar as the hot air above the contents cools. When the juice bottle is hit from the bottom, the liquid contents of the bottle moves slightly, compressing the air at the top of the juice, or pushing on the metal lid. This causes the metal vacuum indicator top to flex in and out which is what makes the noise we hear. It does not matter which end of the bottle you hit, the lid still flexes making the noise.
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