A key for operating a mechanical lock, which key includes a head portion for holding by a user, a lock-engaging portion for inserting into the lock for operating the lock by the application of a turning force, and a joining portion between the head portion and the lock-engaging portion and fixing the head portion to the lock-engaging portion. The joining portion includes a shape memory material, which reversibly deforms without breakage when a first turning force on the key is exceeded, where the first turning force is lower than a second turning force. The second turning force being a turning force which if exceeded causes breakage of the lock-engaging portion or the head portion. The shape memory material may exhibit a superelastic effect in which the shape is restored when the turning force ceases, or shape memory effect in which the shape is restored on heating above a state-transition temperature.