Where did the expression “can of corn” originate as a description for a lazy fly ball?

The common belief is that the phrase comes from the old grocery store practice of using a pole to tip an item (like a can of corn) off of a high shelf so that it tumbles down, allowing the grocer to make an easy catch with his hands or apron. Another theory came from announcer Bob Prince, who said, “it’s as easy as taking corn out of a can.”