How did Mary Shelley come up with Frankenstein?

Mary Shelley was inspired to write Frankenstein after a dream she had on June 16, 1816. The dream is often described as follows: She saw a pale student of unhallowed arts kneeling beside the thing he had put together. I saw the hideous phantasm of a man stretched out, and then, on the working of some powerful engine, show signs of life.

Percy Bysshe Shelley, her future husband, encouraged her to write about her dream. She began writing and finished the manuscript for Frankenstein, or the Modern Prometheus, in May 1817 at the age of 19.