Preschoolers will recognize themselves in Pigeon, a big-eyed blue bird that really, really, really wants a puppy. He's wanted it "forever! At least since last Tuesday."
Pigeon speaks directly to the reader from page one. "Oh, hello. How are you? I'm fine, thanks for asking." He proceeds to tell you how much he wants a puppy, how he'll take care of it by watering it once a month. Pigeon gets emotional when he concludes that you don't want him to have a puppy. He throws a tantrum: "I WANT A PUPPY! RIGHT HERE! RIGHT NOW!"
Mo Willems uses speech balloons to show that Pigeon is indeed speaking to the reader, and he changes the size of the type to show Pigeon's changing emotions. When the bird sulks, the type size is small, as if Pigeon is muttering to himself: "You just don't understand." And when the puppy arrives on the scene with a friendly "Woof?" Pigeon's terror shows with a bold, black scrawled "Aaaaaggghhh!"
Of course Pigeon changes his mind about wanting a puppy, and decides what he really, really, really wants is... a walrus.