From 11:00PM PDT on Friday, July 1 until 5:00AM PDT on Saturday, July 2, the Shmoop engineering elves will be making tweaks and improvements to the site. That means Shmoop will be unavailable for use during that time. Thanks for your patience!
In theater, the fourth wall is that invisible forcefield around the stage that keeps actors in their own little world, and separate from the audience. Actors on stage—and even on screen (we're looking at you, Jim Halpert)—sometimes break the fourth wall for dramatic (or comedic) effect, and directly address the audience. The fourth wall didn't really develop in the theater until the late 18th and 19th centuries.