"Don't eat it!" said the guard," or you can never leave the underworld." But if turned out that she had already eaten 6 pomegranate seeds. They decided it would take the matter to Zeus.
Hades said that he only captured her because he was lonely, but Demeter thought that was no reason to capture her daughter. After much more arguing, they finally agreed that because Persephone ate 6 out of 12 pomegranate seed that she would spent 6 month in the underworld and 6 months in the normal world.
So for 6 months the world would be sunny with lots of flower and for the other 6 the world would be cold with dying plants.