To most fish, holding a mirror in front of them doesn't really do anything. But if you hold a mirror in front of a Betta fish, the fish will start acting differently. Just like when your dog, cat, or other pet sees itself in the mirror, it thinks there is another animal and the pets try playing, barking, or fighting with the reflection. For a Betta fish, it is the same concept. The Betta fish is a fighting fish, so when it sees itself in the mirror, it prepares to fight its reflection thinking it's another fish.