There seems to be a growing tendency on the 'net to answer a question (or otherwise contribute an item of information to the conversation) with another question of the form :
“Can you say <insert answer to original question here>?”
For example,
Person A: I wonder where rain comes from?
Person B: Can you say clouds?
Where does this expression come from? And am I the only person who finds it intensely annoying?
Mister Rodger’s Neighborhood, ostensibly. Although yours truly (a fan) never heard him utter those words on one of his shows.
In Robin Williams’ early days, he was fond(as were a lot of comedians) of poking fun at the Presbyterian minister’s accent and slow way of speaking, often giving him a fey personality for the sake of a joke.
To whit: “Let’s put Mr. Hamster in the microwave! See? Ya have a balloon now! Can you say ‘Pop goes the weasel?’ I knew you could!”
I would go along with Quasimodems answer except to add that the greatest popularizer of that phrase was probably Eddie Murphy in his “Mr. Robinson’s Neighborhood” sketch on SNL. In both cases (Rogers and Murphy) the context and meaning were entirely different from the use you quote. A typical Fred Rogers use would be more like.
“Thi fish are in their aquarium. Can you say that? Aquarium?”