“Before he was many days old he was not only looking like a mouse but acting like one, too—wearing a gray hat and carrying a small cane.”

Stuart does not, in fact, act at all like a mouse. This little piece of ironic humor sets the tone for the story, establishing a blend of fantasy and reality. It tells the reader not to take the story too seriously.

The author uses simile to imply that while Stuart looks like a mouse, he is not one. Carrying a cane suggests that he walks upright, and the illustrations by Garth Williams in the first edition depict Stuart as a boy with the head and tail of a mouse.

“‘How was it down there?’ asked Mr. Little, who was always curious to know about places he had never been to.

‘It was all right,’ said Stuart.”

Stuart is always reticent about his adventures. He often goes places and does things by himself that his family knows nothing about. In that, he is like an imaginative, introverted child who lives a rich and vivid fantasy life that is invisible to the people around him. Stuart’s father—part of the large, adult world—wishes he could share some of Stuart’s freedom to explore exotic places and have adventures.

“[Lifting the sticky piano key] was no easy job for Stuart, as he had to crouch down between the felt hammers so that he wouldn’t get hit on the head. But Stuart liked it just the same: it was exciting inside the piano, dodging about, and the noise was quite terrific. Sometimes after a long session he would emerge quite deaf, as though he had just stepped out of an airplane after a long journey; and it would be some little time before he really felt normal again.”

“Terrific” in this context means enormous, intense, or even terrifying. However, there’s also a connotation of wonder. The adventurous Stuart is stimulated by the noise and excitement within the piano.