Water, water, everywhere
When I was at school I discovered that if you put a piece of card over a full glass of water, you could turn it upside down and the card would keep the water in the glass. You could then place the glass - still upside down - on a table and withdraw the card. This effectively booby trapped the glass, so that anyone who picked it up would end up with a table full of water.

The vague link to vandalism is that I went through a phase of doing this with the paper cups at McDonald's, and had many a gleeful moment imagining the denoument of the hapless tidying-up person and the mess they were going to make.

Needless to say I was far too much of a coward to actually stick around and see what really happened. So I mention it here, first of all because it's quite a fun trick to actually do, and secondly in the hope that someone braver than me might try it out in McDonald's and let me know how it goes.
(richardm, Wed 13 Oct 2010, 16:27, 2 replies)

Incidentally...
... my initial experiments were done in school, and the resulting pillorying by the dinner ladies meant that I discovered the useful information that you could disarm the trap by sliding the glass to the edge of the table and draining it into another glass.
(richardm, Wed 13 Oct 2010, 16:29, closed)

I haven't done anything like this
But when I was at school, I used to un-screw the cap on the salt slightly so the next person got a plate full of salt on their chips (pre Jamie Oliver)
(bROKEN aRROW PUA HVI Master, Wed 13 Oct 2010, 16:50, closed)