In the traditional classroom, the teacher is the sole source of all knowledge. The teacher is the "sage on stage," standing in the front of the classroom and imparting her knowledge, mostly a steady stream of facts. The role of the the constructivist teacher is "the guide on the side." She plays a supporting role in the classroom, choosing topics and setting goals, but allowing students to decide how to reach those goals and create their own learning priorities.
In the traditional classroom, students have little or no interaction with each other. All discourse flows from the teacher. Students sit in rows facing the teacher and the blackboard. Students listen to the teacher and take notes or do workbook exercises as instructed. In a constructivist class, students are more likely to sit at tables and face each other. There is a great deal of interaction between students as they work in groups in order to complete assignments.
A traditional history class emphasizes people, places and dates. The teacher and the textbook tell students which facts are important enough to memorize. A test, most likely multiple choice, measures their success at memorizing those facts. A constructivist history class, on the other hand, begins with a discussion among students, who formulate their own questions about the past. The opening questions, such as "How did Colonial restrooms work?,"may appear to be trivial, but they lead to further study and research into more profound questions about colonial city planning and sanitation.
A traditional science class starts with a textbook and a teacher who emphasizes and explains certain aspects of the textbook. Students memorize whatever the teacher points them toward --- physics formulae or the periodic table, for example. In a constructivist science class, the lesson is opened with concepts students are already familiar with, such as weight. The teacher gives students a balancing scale and some clay and asks them to create their own system of weight measurements. Students will extend their previous notions of weight and balance, eventually seeing their central role throughout science.