The sentence that states the central message you want to communicate is the

thesis statement
topic sentence
conclusion
hook

Thesis statement.

Explanation:

A thesis statement is a concise formulation or summary of the main idea, point, or topic that you wish to elaborate throughout any form of dissertation. It should, ideally, be expressed in a single sentence and this is recommended to be placed in the introductory section —depending on your rhetorical skills, your argumentative line, and academic traditions being followed, it is recommended to place it either at the middle of the introduction or at the end of it.  

well basically since ima beginner people will answer any subject "right away"and if you get more points you could ask more questions or if you answer someone's question you get points

  this riddles' answer is library.

  the "leaves" are a reference to the books' pages that the library contains; the branches are a network of libraries that connect multiple libraries' collections, resources, and items in a system of a common database, integrating them; the branches' organization are a useful management tool for coordinating the multiple collections many libraries have.