How do lines 15-21 act as a flashback? What clues do they give about the rest of the story?

Lines 15-21 act as a flashback in that the writer is remembering details of a past experience. The author mentioning a 'needle of hot silver, a thread of fire' alludes to the fire that is described later in the story. The fact that the memories were brought by a 'whiff of smoke' is also a strong indicator that the author had an experience with a fire.

Explanation:

Lines 15-21 act as a flashback in that the writer is remembering details of a past experience. The author mentioning a 'needle of hot silver, a thread of fire' alludes to the fire that is described later in the story. The fact that the memories were brought by a 'whiff of smoke' is also a strong indicator that the author had an experience with a fire.