Plaster of Paris is a
pasty mixture that is used for body
casts,
sculptures, and most noticeably for covering
ceilings and
walls.
Plaster of Paris' compound is made up of gypsum (or possibly lime), water, and sand. Sometimes other elements such as nylon fibers are added to the mixture to improve stability.
After mixing the dry elements, you will be left with a white powder that can then be mixed with water. Once mixed with water, a chemical reaction will be invoked, and the mixture will heat up. Once cooled, it takes on the form of a solid.