You need to know two things to understand the humor of this situation.
1) Jack the cat has very sharp claws. Even when the claws are trimmed, they are still sharp. There’s just no way around it.
2) In anticipation of receiving his first pocketknife, Steven is learning the rule of not releasing a sharp object until the recipient has said, “Thank you.”
Steven had Jack and was passing him to Angela. After she had Jack in her lap, Angela declared, “Sorry, I should have said, ‘Thank you.’ Fat Jack is a sharp object, isn’t he?!”