Guppies—those unsuspecting 2.5-centimeter-long fish that fill aquariums—guard their food by biting and body checking other fish. If a not-so-subtle tail whack doesn’t get the message across, the eyes will. That’s because guppy irises turn from silver to black when they get aggressive, The Atlantic reports. Using a robotic fish to test guppy gumption, researchers found that the fish were likelier to go after food protected by a smaller, silver-eyed robofish than a black-eyed one. That suggests that black eyes are a true signal of aggression in guppies, they report today in Current Biology. The discovery also means eye color is a meaningful form of communication, telling food looters not just to stay away, but also that there is something worth fighting over.