A sentence starts out like...

a baby starts out into the world. It's vulnerable. It's bare. It's sometimes kicking and crying, and other times peaceful and sleepy. It's the writer's thought, the writer's child. It's passion displayed in a physical form. As the sentence grows, it becomes more refined. More gets added to it, and it will go through different attitudes and changes. The writer will pour his or her heart into it, working it and reworking it, striving to make it the best it can be. Sometimes a struggle, sometimes a joy, the simple starting sentence grows into a paragraph, and from there into a complete thought. Taking great care the whole way, the writer watches it grow, makes sure it stays healthy and headed in the right direction, until finally the writer is ready to let the complete, grown thought head out into the world, sure that it can stand independently.