When does a joke become a dad joke?
When it leaves and never comes back.
The Dad Joke Transformation
Explanation
This is a play on words. The punchline implies the joke turns into a 'dad joke' when it 'leaves and never comes back' which is a reference to the common stereotype of fathers who go out for cigarettes, milk or other thing and never come back. It also aligns with the perception of dad jokes being corny and not clever enough to come back to mind again.