The dwarves arrive at the Lonely Mountain and position themselves outside the hidden door into the caverns beneath it, where the hoard of treasure-and Smaug-await them. As Bilbo is sitting on "the doorstep" (as the company calls it), thinking about how to enter, he sees a thrush knocking a snail against a stone. He remembers the runes on Thorin's map (see Chapter 3) and watches a single ray of the setting sun's light pierce the clouds and illuminate a keyhole. Thorin unlocks the door with the key he has been carrying. The door described on Thorin's map opens, only to reveal darkness beyond.
In this highly descriptive chapter, readers again see how Bilbo functions as a hero. He remembers the runes on the map, and only he is observing his surroundings carefully enough-for he is undistracted, as are Thorin and the dwarves, by dreams of wealth-to see how those runes are fulfilled. Tolkien also uses some irony in this chapter. In Chapter 1, the dwarves had, somewhat cynically, told Bilbo that his job as burglar would involve sitting on the doorstep and thinking of a way to enter the Lonely Mountain; now, as the hobbit points out, Bilbo does exactly that, and thus discovers the solution to that problem. Readers sense that Bilbo surprises even himself-as Gandalf had said he would-with his usefulness to the expedition.
The Hobbit: Novel Summary: Chapter 11