I love Mo Willems. I really, really do.
And you know what? I love Elephant and Piggie. These books work on so many levels. First and foremost, they work well in their job, which is "easy reader". They ARE easy to read... but not too easy. And while the illustrations do illustrate the plot (and very funnily, I might add), they don't give away the plot. Children have to actually read the words to know what the book says instead of guessing from the context of the pictures. And best of all, the books are SO funny and SO over the top that even the most reluctant reader has to ham it up when reading, they can't help it! So when reading, kids are avoiding that terrible, horrible drone-of-boredom so many of them cultivate.
These books also function well as read alouds because they're so funny. Try reading Dick and Jane aloud. You can't do it. (If you're going to buy reprints of Dick and Jane, do so, but don't read them to your kid. They don't work for that.)
And, really, they're funny for grown-ups too. Don't tell my nieces, but I buy these books for ME, even if I store them on THEIR bookcase!
So, this particular book, I am Going!. Everybody reading the title knows that Piggie is not really going away for good. Gerald will still have somebody to skip and play pingpong in a funny hat with. But Gerald reacts in an incredibly melodramatic fashion (we all saw THAT coming too) until he finds out: Piggie is GOING to have LUNCH. (Punchline: They eat together. "Just like yesterday!")
Best part? Has to be Gerald's forlorn pleas that Piggie should go "next month" or "next year", said while holding a Piggie-themed calendar.