As a matter of fact, you can use anything for self-defense ranging from an iron rod to a BB gun. However, some people prefer a BB gun for self-defense because of its uncanny resemblance to an actual weapon, thus making it far more fearsome. It will also cause enough damage to your harasser to knock him down until you can flee the scene. But don't worry if you don't have a BB gun; you can simply make do with anything. Even a knee-jerk reaction would suffice.