Imagine that you’re building a house. You’ve already built the frame, so you have plain, wooden beams in the shape of a house, but it doesn’t look like a home yet. To make it look livable, you’d need to fill out the rest: add some drywall and shingles, paint the walls, install the windows, and so on. That wooden frame may make up the skeleton of the house, but it needs the pieces you add around the frame to appear complete.
