They live in "schools." Many times they will wait for prey
to come to the shallow water of the river. Then the large
group of piranhas will attack. These large groups are able
to kill large animals... Their lower teeth fit perfectly
into the spaces of their upper teeth, creating a tremendous
vice-like bite... Piranhas are attracted to any disturbance
in the water.
[ http://www.animalsoftherainforest.com ]