My dad always says he knows everything.

This joke tickles our funny bone through a classic case of irony. The setup builds up the dad as a fount of all knowledge, someone who claims to have an answer for everything. But then, the punchline delivers a swift and humorous deflation of that claim. His immediate admission of ignorance about dinner, coupled with the classic parental redirection to "ask your mother," creates a delightful contradiction that we can all recognize and chuckle at.

The humor also stems from tapping into a very common and relatable family dynamic. For generations, in many households, the mother has often been the primary planner and orchestrator of meals, even if both parents contribute in other ways. So, while the dad's claim to omniscience is a playful exaggeration, his inability to answer the dinner question, and his default response, hits close to home for many, making the joke land perfectly. It's a gentle poke at the division of labor and perceived knowledge within a family unit.