Think of it like a dry, thirsty sponge that only needs a small amount of 'water of crystallization' to keep its crystal shape stable until you mix it with more water to set it permanently.

PoP (Plaster of Paris) = Half H-two-O (H2O)

Plaster of Paris is produced by the partial dehydration of gypsum (calcium sulphate dihydrate, $CaSO_4 \cdot 2H_2O$) at a controlled temperature of 373 K. At this temperature, the gypsum loses $1.5$ molecules of water to form the hemihydrate. When mixed with water, it rehydrates to form hard gypsum again, which allows it to set as a solid structure.

Always remember the difference between Gypsum ($2H_2O$) and Plaster of Paris ($\frac{1}{2}H_2O$); this is a classic high-yield question in competitive chemistry.