Why did Leela's mother slap her in Leela's Friend?

Leela's mother slapped her because she lost her chain.

Leela's mother noticed that a gold chain Leela had been wearing was missing. "Where is your chain?" Leela looked into her shirt, searched and said, "I don't know." Her mother gave her a slap and said, “How many times have I told you to take it off and put it in the box?”