The teeth of piranhas are as sharp as scalpel blades,allowing a shoal of these fish to rip apart any unfortunate animal in the water in a matter of minutes, leaving just the bare bones.
Although piranhas can be induced into a feeding frenzy by the presence of blood in the water, this usually only happens under certain conditions—such as when the fish are extremely hungry, if their pool is drying up, or if their prey is injured or struggling to stay afloat. The size of the animal will not stop the piranhas; even horses are likely to be attacked by these fish, and dragged down as they wade into the water. Yet not all piranhas are dangerous, nor is size necessarily an indicator of the risk they pose. Some of the larger species, such as the green tiger piranha, which can grow 17 inches long, are no threat to people. On the other hand, the largest species, the blacktail or Saõ Francisco piranha, is definitely dangerous.
