Disclaimer: Not mine.
Amortentia
"Can anyone tell me what this is?"
Tom raised his hand. "It's Amortentia, sir. The most powerful love potion in the world. It can be identified by its smoke, which rises in spirals, and its pearly sheen."
"And what other property can one use to recognise this potion, m'boy?"
"Its smell, professor. It smells different to each person, as its aroma takes the form of whatever attracts us."
"Excellent! Ten points to Slytherin."
Tom sat back, feeling pleased with himself. The aroma surrounded him, it smelled familiar - what was it? Something fruity ... Citrus? No ...
Pineapple?
Yes, pineapple. More specifically the crystallised variety.
But Tom didn't like crystallised pineapple. In fact, the only reason he was able to distinguish its scent was that he had bought a box of it for Slughorn the other day, because he liked it so much and -
" ... May I be excused, Professor?"
D'you get it? Eh? Eh?
Crystallised pineapple, as far as I can recall, does smell different from normal pineapple - I used to really like it a few years ago.