The piranha, which lives in rivers in tropical Central and South America, is the fiercest of all freshwater fish. Each fish is only about 12 in (30 cm) long, but a shoal of hundreds attacking together can kill and eat a much larger animal in seconds. The piranha's weapons are its extremely sharp triangular-shaped teeth, which it uses to chop flesh from its victim. But not all piranhas are dangerous killers. Some species feed only on plants.