Why do bananas get brown spots?

they turn yellow because there ripe now....and theres like pigments in them or something like that, which go over the green and the green goes away and then it turns yellow :)

It is due to a defense mechanism in the fruit, which is activated when the fruit is cut from the tree.

There are three main factors involved in the ripening and browning process, they are: enzymes, oxygen and phenols. Enzymes are protein-like substances that speed up or initiate chemical reactions. Phenols are defensive mechanisms the plant uses to fend off insects and microbes. The specific enzyme is called polyphenoloxidase. When the banana is attacked, the phenols are activated in the presence of oxygen by this enzyme. In this process the brown color is produced as a side effect. The first time the banana is attacked is when it is cut from the tree. Exposure to cold air, certain gases, and physical damage of the fruit will hasten the process.