Why is it difficult to straighten a bent metal coat hanger?

Straightening a bent metal coat hanger is difficult because the metal has undergone plastic deformation, meaning it has permanently changed shape and lost its original structure. The forces applied during bending cause the metal's crystalline structure to rearrange, resulting in internal stresses. Additionally, the thin and flexible nature of a coat hanger makes it challenging to apply sufficient force uniformly without risking further bending or breaking.