Definition of Metaphor
At the end of Chapter 11, Thorin manages to open the side door into the Lonely Mountain. Tolkien uses a provocative simile and metaphor to describe the entrance:
A door five feet high and three broad was outlined, and slowly without a sound swung inwards. It seemed as if darkness flowed out like a vapour from the hole in the mountain-side, and deep darkness in which nothing could be seen lay before their eyes, a yawning mouth leading in and down.
In Chapter 11, Bilbo and the dwarves spend days searching for the side entrance into the Lonely Mountain, to no avail. When Bilbo finally sits down to "think" about the problem, Tolkien includes a detail that serves as an important metaphor for Bilbo himself:
Unlock with LitCharts A+[...]I am afraid he was not thinking much of the job, but of what lay beyond the blue distance, the quiet Western Land and the Hill and his hobbit-hole under it.
A large grey stone lay in the centre of the grass and he stared moodily at it or watched the great snails. They seemed to love the little shut-in bay with its walls of cool rock, and there were many of them of huge size crawling slowly and stickily along its sides.