This was when i was taking my 'O' Level exams in Ghim Moh Secondary School. We were lining up two by two outside the chemistry lab waiting to go on for our chemistry practical exam when...
Me: *sniffs the air* HEY! I SMELL ONIONS!!!
(this was followed by great flurry of classmates flipping through notes to study onion cell structure, etc)
Outcome: we were tested on osmosis and diffusion, using onions to work with.
Then, while we were lining up outside the physics lab, one of my guy friends Ridzuan (the class joker) suddenly jerked his head up, stiffened, with eyes large like saucers said,
"I smell, i smell... PENDULUM!"
I could've died laughing.
Outcome: we REALLY were tested on some concept (gravity, methinks) using the bouncy metal spring pendulum thingy!