Part of the show Naked Q&A and The Life of Benjamin Franklin
Daniel in Israel asked:
My children can make the screw tops of their plastic soda bottles 'talk'. You wet them and place them upside down on the opening of a partly filled soda bottle. If we don't wet the cap, it doesn't work. If we hold the bottle with both hands without squeezing, it 'talks' faster, even after we let go. Why is this?
I guess what's happening here is that you've got a half-filled fizzy drink bottle and inside that drink you've got a lot of dissolved carbon dioxide. That will slowly be boiling off and you can see the bubbles coming up all the time. The gas will be escaping through the little gap at the top. If there's nothing sealing that gap, then the air will escape slowly all the time. If you put some water on the lid, then it'll seal it and cause the pressure to build up a bit. The gas will then pop out and reseal again; seal and pop, seal and pop. This is probably what you're hearing.