Piranhas are ferocious and deadly!!!
Piranhas do have sharp little teeth and can give you a nasty nip. They do sometimes swim in large schools, but only to defend themselves against natural predators, such as river dolphins. Piranhas are predators of smaller fish and shrimp. They do have to be kept in even-numbered groups in aquariums to stop them ganging up on an odd one out.
There are no records of piranha attacking and killing live humans. Only few examples of bites from careless handling of piranha out of the water demonstrate the potential for injury. They see color. Brazilian fishermen use a red piece of cloth or piece of red meat to fish with and piranhas do react to this color. They do not habitually attack humans. In fact, humans eat more piranhas than the other way around!
It is a shame that we have made the piranha a larger-than-life monster.