Final answer:
Simile. The phrase 'as comfortable as a bed of nails' is a simile, a figure of speech that compares the uncomfortable rocking chair to something universally recognized as painful, emphasizing its discomfort.
Step-by-step explanation:
The sentence "The old wooden rocking chair that Lauren brought home from the rummage sale was as comfortable as a bed of nails." is an example of a simile. A simile is a figure of speech that compares two different things in an interesting way, usually using the words 'as' or 'like.' This simile compares the discomfort of sitting in the rocking chair to lying on a bed of nails, implying that the chair is very uncomfortable. In literature, similes are used to create vivid imagery and give the reader a deeper understanding of the object or situation being described.