[New thread because thread hijacking is evil] The following one is IMHO lovely enough to be listed in "The Flying Circus of Physics" by Jearl Walker. You need two full beer bottles of the form shown here: https://de.wikipedia.org/wiki/Jever_%28Bier%29 Holding both gently at the neck, bang (gently!) the bottom of one to the wall of the other (Prost!). This results in a nice ringing "bing". Now sip increasingly more beer out of both. As long as there is beer in the neck, nothing changes bing-wise. But as soon as there is air in an appreciable part where the neck widens to the full diameter, there is a drastic change: no more nice bing, just a dull sound that dies off immediately. Discuss! P.S.: technically, this experiment should work with liquids other than beer. But let's not get silly.