Bettas are well-known for their aggressiveness. When a betta sees a rival male, both will flare their fins and tear each other up. In the wild, they don't usually kill each other. Instead, the loser swims away. However, in a small aquarium, the loser has no where to go so he is constantly getting picked on until he dies, which is why you can only have one male betta per aquarium. On the contrary, it is possible to keep multiple females in a large aquarium, as long as there are more than 4 or 5. They may fight each other for a few days, but its important to leave them alone because they are setting up a pecking order. As soon as that is over, the fighting will stop.
Flaring is used by male bettas as a form of intimidation against other male bettas. Flaring bettas stretch their gills and their fins to make them look twice as big and they do look impressive. One way to make them flare is to hold a mirror in front of them. They'll think that they are seeing a rival and quickly stretch out their fins. Just make sure you don't leave the mirror for more than 5 or 10 minutes so that they don't get tired.
Bubble nests are made by the male when he is ready to mate. This is a good sign and it means that they are healthy. It is where the eggs will be placed and where they will hatch. Once the eggs are in the nest, the male will guard it until the babies hatch. The size of a typical bubble nest is a about a couple inches in width and length.
One of the reasons bettas are so cool is that each one of them are unique. Additionally, they can recognize their owners. They often swim to the front whenever I come into the room. It's pretty cool. They can even do tricks like eat eating off of your hand, following your finger around the aquarium, and flare on command