How do seeds grow?

Question:yes you need sun and water...?? but thats crazy! i just planted sunflowers and the next day (or so its seems) BOOM! can someone explain to me exactly WHY.

Answers:
Inside the seed is a fully developed, self contained embryo consisting of a stem-like axis bearing one or two cotyledons (the first leaf-like structure), the radical (root-like structure) and endosperm (stored food/energy for initial growth). Under the proper environmental conditions, germination is the splitting of the seed coat that allows the embryo to unfold and start its growth process fueled by the internal stored food of the endosperm. It may all look like it happens overnight, but that seed and its embryo took weeks to develop on the host plant, long before germination ever took place.