"A person cannot teach what he or she does not know." This statement makes it clear as to why it is critical that teachers have a solid grasp of their content knowledge. Teachers must have a good command of a subject in order to guide student learning. They need to also be able to make connections withing and across their discipline for their students. Knowing the content of an area is not limited to factual information of a subject, it also includes concepts, principles, relationships, skills in analyzing the facts and concepts and prerequisite relationships. Students should be able to look to their teacher as a source of information about a particular subject.