Plaster of Paris should be stored in a moisture-proof container. Explain why?

(N/A) Plaster of Paris $(POP)$ should be stored in a moisture-proof container because it is a powdery substance that reacts with atmospheric moisture to form a hard,solid mass known as gypsum.
The chemical reaction is as follows:
$CaSO_4 \cdot \frac{1}{2}H_2O + 1\frac{1}{2}H_2O \rightarrow CaSO_4 \cdot 2H_2O$ (Gypsum,a hard solid).