What is "petty theft"? Why did he suddenly mention this?
Native speaker’s answer
Rebecca
In legal terms, "petty theft" refers to the act of stealing property or belongings that do not have a high value. The boy's mother is feeling a bit down, so he tries to cheer up her up by reminding her she has a Ph.D, until he realizes that the name of the certificate is not hers - indicating that she stole it. Ex: The suspect was arrested on suspicions of petty theft. Ex: The man was charged with petty theft after he stole a radio.