4-year-old's joke: What is a duck's favorite snack? Peanut butter (context in comments)

We told my four-year-old a joke: What is a duck's favorite snack? Quackers! HA!

He asked, "Quackers?" confused "Like, peanut butter and crackers?" "Sure, like peanut butter and crackers."

runs into other room, calling his grandfather "Pop Pop! What is a duck's favorite snack?" "Peanut butter" falls over laughing at his own joke