Two I remember from way back (as a trainee)
The guy responsible for us, Brian, had a big blue spotted mug - this was in the days when we had a tea lady - and when he had a day off, one of the other trainees got hold of his mug put about an inch of water in the bottom, then ran freshly mixed silicon rubber compound over it. This was shiny white, almost perfectly matching the colour of the mug. Once completely set he made two tiny holes in it, turned it upside down, and trickled compressed air into one hole, forcing the water out of the other. That done he sealed the holes with another couple of dobs of silicon rubber.
The following day we all managed to be in sight as the the poor instructor had his mug filled by the tea lady. We didn't have long to wait, as the heated air expanded and tea started flowing over the top of the mug - much to the instructors astonishment. Finally the silicon rubber ruptured and there was a mini tea volcano, to the merriment of all.
Much later the guy's mug was accidentally dropped and broke (concrete floor) and the same trainee very carefully put it back together with water soluble adhesive. So this time the instructor suddenly found himself holding just the handle with a bit of the mug, and a desk awash with tea.
Actually we bought him a new mug afterwards for being such a good sport.