The most sophisticated of these attempts, and the ones that show up in the research as the most effective, combine many of these techniques and strategies within a rich multi-faceted educational framework using the model of direct instruction as a delivery technique for the thinking skills and mental habits taught. Strategies for thinking skills and important habits of mind are made explicit, students then use these to think about important elements of curricular content, and a metacognitive layer is woven throughout. This model has now been refined through considerable instructional practice in a very large variety of classrooms.