Rat in a Skinner box, an experimental space used in behavioral studies. Developed by Burrhus Frederic Skinner, the animal within the box can be studied while isolated from variables and influences in the environment and subjected to specific conditions. The box contains a lever and a small tray outside the chamber. This lever is pressed by the animal to gain an award that is dropped into the tray, such as pellets of rat food, or to avoid a form of punishment, such as an electric shock. Animal testing, also known as animal experimentation, animal research, and in vivo testing, is the use of non-human animals in experiments that seek to control the variables that affect the behavior or biological system under study. In the USA the numbers of rats and mice used is estimated to be between 20-100 million a year. Rats are widely used for physiology, toxicology and cancer research, but genetic manipulation is much harder in rats than in mice, which limits the use of these rodents in basic science. Animal rights organizations question the need for and legitimacy of animal testing, arguing that it is cruel and poorly regulated or that animals have the intrinsic right not to be used or harmed in experimentation.