Children progress through several stages of writing development from scribbles to full sentences and paragraphs. In the early stages, children are exposed to print through environmental materials and learn the mechanics of writing. As they develop orthographic knowledge, they progress from random scribbles to controlled scribbling, mock letters, letter writing, and invented spelling. Throughout elementary school, children's writing expands to include more complex sentences, paragraphs, narratives, and genres as they learn grammar, spelling, and the writing process. By middle school, students are expected to write multi-paragraph compositions and refine their skills.