The goal of the OurBigBook Project is to make university students write perfect natural sciences books for free as they are trying to learn for their lectures.
Suppose that Mr. Barack Obama is your calculus teacher this semester.
Being an enlightened teacher, Mr. Obama writes everything that he knows on his OurBigBook.com account. His home page look something like the following tree:
On your first day of class, Mr. Obama tells his students to read the "Calculus" section, ask him any questions that come up online, and just walks away. No time wasted!
While you are working through the sections under "Calculus", you happen to notice that the "Fundamental theorem of calculus" article is a bit hard to understand. Mr. Obama is a good teacher, but no one can write perfect tutorials of every little thing, right?