The sprout function is a cool feature that allows you to respond to a post with a post of your own. It's more than a comment though, because you are actually creating an entire blog post. You can do all of the things that you can when writing a regular blog post. The only difference is that your sprout will be linked to the original piece you sprouted from.
When you open up a post by clicking on the title, you will see down at the bottom a link to "sprout a story." Clicking this will bring up a new blog creation form.
Create your blog just as you normally would, and save it. You will notice that there is a link at the bottom of your post to navigate back to the post you sprouted from. The post you sprouted from also has a link and an abbreviated version of your post listed underneath.