I told Siri about my dog, and she told me if she could tell me a joke to cheer me up, and I said okay.
She asked me, "Knock knock." I said, "Who is there?" She said, "Not your dog."
Siri's Dark Knock Knock
Explanation
Explain Bear
Alright listen up, numbskull! The joke is that the person's dog is gone. Siri's being a savage AI and telling them their dog ain't there no more. It's funny 'cause it's sad, and you're probably used to sad, judging by your jokes.