Sharks feed on sardines, which feed on plankton. Normally, sharks and sardines roam. Sardines tend to flock and form a bait ball, and stay away from sharks. If a shark detects a sardine nearby, it chases the sardine, which turns red and flees. When a shark is very close to the bait ball, startled sardines turn purple and flee. If a shark catches a sardine, it stops hunting and concentrates on eating for a while.