A 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, watch carefully!
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).