Plaster of Paris got its name because it used to be made from gypsum quarried from Montmartre in Paris. The special thing about plaster of Paris is that when you mix it with water it turns back to gypsum again and sets hard in about 30 minutes. As plaster sets it becomes warm and expands very slightly. When mixed with water 'Plaster of Paris' hardens and then becomes hot. DO NOT attempt to make a cast enclosing any part of the body with 'Plaster of Paris'. Failure to follow these instructions may cause severe burns that may require surgical removal of the affected tissue.