One of the first jokes I heard in Spanish

[Marcus]

Un pez pregunta a su amigo, (One fish asks his friend,)

"¿Qué hace tu padre?" (What does your dad do?)

El otro dice: (The other fish says:)

"Nada." (Nothing)

Nada means "nothing," however, it's also one of the conjugations for the verb nadar which means "to swim." Re-translate this joke with that in mind.