Rioting is another form of violence. It is group violence. Rioting presents a direct threat to the security of the institution and the inmates in it and is often met with reciprocal force by the staff and administration of the prison. Prison riots have existed as long as there have been prisons, in fact before there were prisons. Virtually every maximum and medium security prison, with any longevity, has experienced some form of rioting by inmates. Rioting, and violence in general, is engaged in by inmates to achieve some end like better food or housing or power, or inmates might riot out of anger or frustration. When violence is used to achieve some end, it is known as instrumental violence, but when it is just an angry outburst, then it is known as expressive violence. Of course, inmates engaged in violence or a riot could be involved for both instrumental and expressive reasons. An inmate who wants to protest the overcrowding of his or her institution may riot to let the world know about the conditions of confinement (instrumental violence), but he or she might also be angry about the effect such crowding has on housing and the ability to sleep and become violent as a means of expressing it. When enough inmates engage in this violence together, it is called a riot.