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5  Einsatz in der Schule
Innerhalb weniger Tage, nachdem die Anfrage auf
veröffentlicht wird, erhalten die beiden allerlei Informationen.
Während einige Antworten eher humoristischer Art sind, gibt es
auch ernsthafte Reaktionen, und zwei Leute setzen sich mit der
Schülerin und dem Schüler direkt per E-Mail in Verbindung. Die fol-
gende E-Mail ist ein Beispiel:
Subject: Re: Spilling of softdrink cans
I have heard about this trick too. There was a difference however.
The way I learned it, it was good enough to tap the can on the
side. The explanation that I was offered goes something like this:
As you shake the can, CO_2 will leave the liquid and a) bubble up
and join the gases above the liquid, b) stick to the walls of the can
as small bubbles. When the can is opened the bubbles on the
walls will expand a lot and force a substantial amount of liquid out
of the can.
By tapping the sides, the bubbles at the walls are knocked loose
and can rise to the surface. The elevated pressure will not go
down, but as the can is opened the pressurized gases can rush
out without pushing liquid in front of itself.
I think this makes sense considering your poor "batting average"
of 40%. By opening immediately, you run the risk that some
bubbles may not have had time enough to reach the surface. On
the other hand, if you wait to long, new bubbles may form on the
sides (what do I know).
By tapping at the top, you only make the sides vibrate slightly (you
had to tap "with quite some force"). Tapping at the side may be
more efficient.
Have you considered doing the experiment using transparent PET
bottles? (Yeah, I know they come in huge sizes and it would make
an expensive mess to splash 1-2 liters of coke around your
student apartment. Still, you could probably make some
interesting observations without having to open the bottle.)
I should add that I have never tried the trick myself and that I would
be interested in hearing about what you find out about it. 
One overly ambitious source of info:
Christopher E. Brennen. Cavitation and bubble dynamics. Oxford
University Press 1995, ISBN 0-19-509409-3