In a submission, I encountered the phrase "can of corn" as a baseball term; hadn't heard it before, so went and looked it up. Apparently it refers to a baseball hit in such a way that it's particularly easy to catch.
John Marshall, the Seattle Post-Intelligencer's "Answer Guy," says that there are several possible origin stories for the phrase. The most accepted one, he says, is this: In olden times, "[...] a grocer would use a stick to tip a can of vegetables off a high shelf, then catch it in his hands or outstretched apron."
A British site, The Phrase Finder, gives a somewhat different explanation: it repeats a story to the effect that the phrase refers to a shopkeeper lightly tossing a can of food to a customer, and notes that it's a can of corn because the outfield is sometimes called "the cornfield."
But the P-I answer sounds better-researched.
Also, the Phrase Finder answer claims that the phrase was first used by announcer Red Barber, but the P-I answer says it was first used in 1896, which was twelve years before Barber was born.
Heh. Clearly you've never been to a game with me, Jed. I tend to announce this frequently.
So a 'can of corn' is a pop-up fly? (I know nothing of sports, nothing.)
I suspect the reason it's corn rather than peas is for the alliteration. Catch phrases in English are alliterative more often than you'd expect by chance. 'By gosh and by golly', 'the Devil and the deep blue sea', etc., etc.