Why should Plaster of Paris (POP) be stored in airtight containers?

Concept- 

Plaster of Paris, often known as POP, is a white powder that is mostly used to decorate ceilings in homes and buildings as well as to make statues, toys, and blackboard chalk. Gypsum is used to make cement, wallboard, plaster of Paris, soil conditioner, and Portland cement, among other things.

POP is stored in airtight containers because they may absorb moisture and get harder and settle down which makes them impossible to use. It should be mentioned that there are three varieties of plaster; the POP is one of them. The other two forms of plaster are cement plaster and lime plaster. Cement plaster is created by mixing sand, plaster, Portland cement, and water. 

Plaster of Paris should be stored in airtight containers because it absorbs moisture (or water) to form a hard substance called gypsum which makes Plaster of Paris useless.