The sore silversmith

Mr. Argent came into the agency. 

“I want to hire you.”

He explained.

Argent’s Silver Shop was a jewellery store and sold fine silver necklaces, rings and such. His prised possession was a pure silver necklace dating back to the 1800s.

“It was stolen.”

During the night before, Mr. Argent’s alarm rose and he saw the glass box guarding the jewellery smashed and the necklace gone. The only thing he found was a penknife. 

“Ernie’s.” Said Jack immediately.

Ernie had a new intruder and Jack duly explained everything to him.

“What? No! I saw Jim holding an old necklace. I bet it is him!”

Jim then came along, holding his old and rusty necklace.

“Ah ha!” said Jack. “Ernie! You are the real stealer!”

WHY?

Because silver doesn’t rust.