Final answer:
A blacksmith places hot iron in cold water to rapidly reduce its temperature, a process called quenching, which finalizes the shape and enhances the metal's properties.
Step-by-step explanation:
A blacksmith places a piece of hot iron in cold water because doing so rapidly reduces the temperature of the iron through a process known as quenching. When iron is heated to a high enough temperature in a forge, its atoms vibrate more intensely and can move around more freely, making the metal malleable. After shaping, immersing the hot iron in cold water transfers the heat energy from the iron to the water, causing the iron atoms to slow down and the iron to become rigid and hard again. This is essential not only to finalize the shape but also can have effects on the microstructure of the metal, enhancing its properties such as strength and toughness.