  "Beer tapping" is a well known prank where a bottle of carbonised liquid
strikes another bottle of carbonised liquid from above, with the usual result
that the lower bottle foams over whereas the upper one does not. Though the
physics leading to the foaming process in the lower bottle has been
investigated and well documented, no explanation to date has been provided why
the upper bottle produces little to no foam. In this article we describe the
reasons for the entirely different behaviours of the two bottles.
