Answer: The puppy’s excitement on her night walk.
Explanation: The topic of the poem is the puppy’s excitement on her night walk. The theme is that the exciting experience of youth improves with age. This theme is supported by details about the puppy. She is happy to dash around, but she gets tired when the walk has taken her “far enough.” The speaker reflects that the puppy “can’t run all night when she is so small,” but when she is older she’ll be “taught” and able to do more.