Some topics seem systematically extremely difficult for students, and conventional methods of teaching usually fail. Topics like this are found at all age levels, including, in physics; matter and density, Newtonian mechanics, electricity, and relativity; in biology: Evolution and genetics. These topics are difficult to learn, because learning them requires students to undergo a conceptual change. Conceptual change contrasts with less problematic learning, such as skill acquisition (learning a physical skill or an algorithm for long division) and acquisition of facts (such as "basic number facts" for addition or multiplication). If there are difficulties in these areas, they are for more apparent reasons such as sheer mass of learning or the necessity of practice to produce quick, error-free performance. The name "conceptual change" embodies a first approximation of the primary difficulty: Students must build new ideas in the context of old ones, hence the emphasis on "change" rather than on simple accumulation or (tabula rasa, or "blank slate") acquisition. Evidence strongly suggests that prior ideas constrain learning in many areas. The "conceptual" part of the conceptual change label must be treated less literally. Various theories locate the difficulty in such entities as "beliefs," "theories," or "ontologies," in addition to "concepts."