Because Zeus did nothing to get Persephone back, Demeter didn't let anything grow. Everything grew cold and sad. Zeus knew he had to do something.
Zeus decided to set up a deal with Demeter.
Very well then, my daughter is strong. I have faith in her and know that she will not eat since she is filled with sadness without me.
Demeter lets have a deal. I will allow Persephone to return to you as long as no food shall pass her lips while she is in the underworld. But you have to promise me that you shall let things grow.
I'll deliver the message to Persephone.
I just want to be with my mom. I refuse to eat cause I'm filled with sadness. No gift will make me happy.
meanwhile, in the underworld.
My love, please eat. You have to be so hungry, you haven't eaten since you get here. What can I do for you? I got you a present, will you please take it?
Even though Persephone refused to take the gifts and said that she didn't want them, she secretly loved it. Part of her mind actually thought that Hades was a good guy and she could get use to it here in the under world.
How thoughtful of him.
But then, a boy from the garden (Demeter turned this boy into a lizard and he ended up in the underworld) offered Persephone pomegranate, her favorite fruit. Before Hermes could give Persephone the message, she ate 6 seeds.
hm... I mean I am extremely hungry, and if no ones here it can't hurt. *Eats 6 seeds*
There's no one here, you're alone. Eat, you'll feel better.
Even though Zeus said that if Persephone ate anything she'd have to stay in the underworld, he thought that he did have to do something. So for every seed Persephone ate she'd have to stay in the underworld for that amount of months. And so Persephone was to stay in the underworld for 6 months and 6 months with her mother. In the time that Persephone was in the underworld nothing would grow and while she was with her mom everything was fruitful. This is why we have summer and winter.
Sorry, but at least I still get to spend time with you, mom.