A piranha is a fish with famously sharp teeth. Don't go swimming in certain South American rivers if you want to avoid piranhas!
These freshwater fish are terrifyingly described as "voraciously carnivorous," meaning they eat meat with great enthusiasm. Humans aren't often bitten by piranhas, but it does happen in some parts of South America. You also can't buy a piranha as a pet in many parts of the world—and if you do, it's best not to get two of them, since the weaker fish is often killed by the stronger one.