Monday, 17 February 2014

The haunted milk bottle

Bottle capI was alone. It was silent. The howling wind which, with torrential rain, has provided the sound track to this winter, was taking a day off. So I could hardly miss the insistent tapping sound coming from an adjacent room. The taps were at a regular interval of a few seconds. Utterly mystified, I entered the room where the raps were coming from but could see nothing that might be their cause.

I was reduced to moving around slowly and listening carefully to localise the sound. Finally, I discovered it was coming from a plastic milk bottle. I looked at it, held it, shook it, but remained baffled. How could an empty milk bottle tap by itself? Unless it was haunted, perhaps? I wondered if I should establish a code of one tap for yes, two for no.

Gradually, I localized the sound to the screw cap. It was tapping periodically as it lifted ever so slightly and then dropped again. Suddenly, the recent history of the bottle seemed relevant. I had, minutes before, removed the bottle from a fridge and emptied it of milk. This was because the contents was beyond its use-by date. I had halfheartedly screwed the cap back on and left it. This meant that the bottle was cold from the fridge and now filled with air. The room was warm. Clearly, as the bottle warmed up, the air expanded and pushed the cap up. After a little gas escaped, the pressure reduced so that the cap fell back, re-sealing the bottle and producing a tapping noise. This sequence repeated every time the warming air built up sufficient pressure to momentarily force the cap up.

This incident reminded me of similar ones I've experienced in the past with bottles removed from a fridge and partially sealed. Instead of a tapping noise, the usual result has been a mysterious whistling noise. The cause, air warming up and expanding, was the same. I suspect the tapping phenomenon is much harder to achieve as it requires a delicate balance of the weight of the cap and just how tightly it is screwed on. The lesson is, when you hear phantom whistling or tapping, it is worth checking any containers recently removed from a fridge!

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