The theme of this powerful poem is racism, and the impact that it has. This is shown through the childhood memory the speaker has of being eight and cycling through Baltimore. Even though he was there from May until December, the only thing he remembers is the one incident that he describes in this poem. One day, when he was riding, "heart-filled, head-filled with glee," he met another boy who said just one word to him, but this incident obviously managed to destroy his happy mood and scar him forever:
Now I was eight and very small,
And he was no whit bigger,
And so I smiled, but he poked out
His tongue, and called me, 'Nigger.'
Note how this action is simple: it involves just one word and an accompanying action. However, it is clear from the way that this is the only memory the speaker has of those long months, that this incident was actually much more than just an "incident": it powerfully affected him and made him aware of the brutal reality of racism and the power that it can have over its victims.