Changes in political and social systems have often been brought about by revolutions. A revolution means a drastic or radical change. A revolution can be the sudden overthrow of an established government or system by force and bloodshed; it can also be a great change that comes slowly and peacefully. People do not usually revolt against a government or a certain system unless they believe that it is no longer possible to live in the old way. Revolutions occur when an existing system becomes unbearable to a vast majority of the people. Other revolutions which have taken place are the industrial revolution, the American revolution, etc.