Page "Eleusinian Mysteries" Paragraph 5

Zeus, pressed by the cries of the hungry people and by the other deities who also heard their anguish, forced Hades to return Persephone.

However, it was a rule of the Fates that whoever consumed food or drink in the Underworld was doomed to spend eternity there.

Before Persephone was released to Hermes, who had been sent to retrieve her, Hades tricked her into eating pomegranate seeds, ( six or four according to the telling ) which forced her to return to the underworld for some months each year.

She was obliged to remain with Hades for six or four months ( one month per seed ) while staying above ground with her mother for a similar period.

This left a long period of time when Demeter was unhappy due to Persephone's absence, neglecting to cultivate the earth.

When Persephone returned to the surface, Demeter became joyful and cared for the earth again.