An enzyme in pineapples called bromelain destroys proteins! If you try to put fresh pineapple slices into your Jell-O, the bromelain will break down the gelatin. If you tried to mix it with Jell-O mix and put it in the fridge, it’ll stay in liquid form instead of turning it to Jell-O. Canned pineapples don’t have bromelain, so you can put those in your Jell-O instead.
Bromelain can also be used to ruin a perfectly good steak. A little bit can be added to tenderize the meat, but if you leave it in the steak for a couple days, it gets really mushy. It has the same effect on human tongues. However, your tongue regenerates cells quickly, so you don’t have to worry about eating pineapple (unless you’re allergic).
You can learn more facts about pineapples, and tips on how to prepare them at Dole's website. Learn more about Bromelain at How Stuff Works.