Epoxides like to react with nucleophiles. Usually, when an epoxide reacts with a nucleophile, the first thing it does is break one of its chemical bonds between the oxygen atom and the carbon atom. This makes the triangular ring pop open. With the ring open, the nucleophile gets stuck onto one of the carbon atoms. On the other carbon, the left over oxygen attaches to a hydrogen atom, turning into an alcohol. This is very useful, because it is easy to change an alcohol into many other functional groups.