This reminds me of a similar stunt we used to enjoy at the dining hall
in my undergrad days. The food service used opaque plastic salt and
pepper shakers with pop-off tops that could be pried off with a knife
blade if you were persistent enough.
PREPARATION (in a restroom nearby): (1) Empty salt ( or pepper) from a
previously 'acquired' container and fill about 1/3 full with
concentrated lemon juice. (2) Place a thin tissue across the opening,
poke it down a bit to form a depression, and fill the depression with
about a teaspoon of baking soda. (3) Cover (from the inside) the
holes of the top with tape of the appropriate color. (4) Replace top
on container and trim visible tissue from around the top.
Carry the device to dining hall (upright and as stable as is
possible... for your own sake).
After discretely placing the shaker on your table (only place it near
to you... see caveat \#1 below), observe the next person to use the
salt (pepper). (S)He will shake lightly at first, then harder as
nothing comes out. Due to the breakdown of the tissue and the pressure
resulting from the classic acid/base reaction, the top will pop off
(quite spectacularly) amidst a shower of foam. Your victim (as will as
everyone around) should have quite a reaction, since one does not
usually observe this type of behavior in a salt (pepper) shaker!
CAVEATS: 1. The top will come off with some force. If the holes are
sealed well, this will happen on about the second or third shake.
Once, though, due to poor sealing, it took about 5 seconds, during
which time our victim started looking at the shaker to examine the
"foamy stuff coming out" of the holes... we quickly grabbed the shaker
from her to direct the top towards the ceiling before it went off. So,
2. The "foam shower" (lemon juice \& soda) may ruin you victim meal...
be prepared to pop for another one.
3. Don't do this if your victim or anyone near ground zero is dressed
up (this joke will flop at board meetings and the like).