How do seeds grow?

Lately Leah has been very interested in seeds. She started collecting them after eating an apple. Her idea was to replant them in our backyard this spring so we would have an apple tree. Then she grabbed pumpkin seeds from our jack-0-lanterns. Tim bought her some sunflower seeds so she could eat them, but instead she put them with her seed collection after eating only a few. The last group of seeds that she has are popcorn seeds. Maybe she'll add cherry seeds next!