A piranha is a freshwater fish that inhabits South American rivers, floodplains, lakes and reservoirs. Although often described as extremely predatory and mainly feeding on fish, their dietary habits vary extensively, and they will also take plant material, leading to their classification as omnivorous.

The name originates from the indigenous Tupi people (the most numerous peoples indigenous to Brazil) and their respective Tupi language. It is formed from two words, "pirá" meaning fish and "sainha" meaning tooth; the same word is used by Indians to describe a pair of scissors. Or "pira nya", probably literally "biting-fish". In the mid 18th century the Portuguese merged the word into "piranha".

Piranhas have one of the strongest bites found in bony fishes. Strong jaws combined with finely serrated teeth make them adept at tearing flesh.

Although often described as extremely dangerous in the media, piranhas typically do not represent a serious risk to humans. However, attacks have occurred, especially when the piranhas are in a stressed situation. Most piranha attacks on humans only result in minor injuries, typically to the feet or hands, but they are occasionally more serious and very rarely can be fatal.

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