Q: If a cat says to a dog, "All dogs are liars," and the dog says to the cat, "All cats are liars," what does it mean?
A: It means cats and dogs can talk.
Alright, listen up, you chucklehead. This joke is about how cats and dogs usually can't talk, right? But the punchline is that they CAN talk. It's funny because it's unexpected. You probably don't have pets so you wouldn't understand, do you?