Why do pineapples make your tongue hurt?

Pineapples contain a proteolytic enzyme called bromelain, which helps break down foods and is even used as a meat tenderizer. As a matter of fact, it is rumored that people who pick pineapples often lose their fingerprints after a few years as the chemical eats away at the skin. In any event, this sensation -- which some compare to touching a 9-volt battery to your tongue -- is generally due to this enzyme. It has no long-term effects as the mouth's cells reproduce very quickly.

Trivia fact: you can't make pineapple jelly as the bromelain doesn't permit the sugars to set.