In her earth-bound wanderings, Demeter came across the daughters of Celeus and wrangled a job as a human family nurse to care for their infant brother Demophon. In addition to more traditional care, Demeter attempted to confer immortality upon Demophon by the frightening technique of sticking the baby in the hearth embers. The baby's mother, Metaneira, discovered Demeter so engaged. Terrified, she stopped Demeter, and in doing so, prevented her son from being made immortal. In return for the disruption of her great gift, an angered Demeter revealed her real, goddess-stature and ordered a temple built in which she would teach her mysteries. To this day, these are known for their location as the Eleusinian Mysteries.
The mythological Demophon is a minor figure. In art depicting the Eleusinian mysteries, there is doubt as to whether it represents Demophon or his brother, a youthful Triptolemus. In addition to this Demophon there is another, a king of Athens and son of the Greek hero Theseus.
Source: Timothy Gantz: Greek Myth