Modality principle: native speakers of the narration language, or when only printed words appear on the screen.[27] Generally speaking, audio narration leads to better learning than the same words presented as text on the screen. This is especially true for walking someone through graphics on the screen, and when the material to be learned is complex or the terminology being used is already understood by the student (otherwise see "pre-training"). One exception to this is when the learner will be using the information as a reference and will need to look back to it again and again.[28] Coherence principle: Avoid using unnecessary content (irrelevant video, graphics, music, stories, narration, etc.) in order to minimize cognitive load imposed on memory during learning by irrelevant and possibly distracting content.[27] Basically, the less learners know about the lesson content, the easier it is for them to get distracted by anything shown that is not directly relevant to the lesson.