What is constructivism? Do we practice this in our classroom?

Constructivism refers to a theory that is more likely based on a way of learning by doing. It says that pupils construct their own understanding and knowledge of the world, through experiencing things and reflecting on those experiences. When we come across something new, we have to relate it with our previous ideas and experience, maybe changing what we believe, or maybe discarding the new information as irrelevant. In order to practice this teacher has to create a better learning environment which can be actively engaged in learning.

This theory is very much against the traditional way of teaching. In the traditional way teacher does everything and provides all the information and the only task for the pupils is to listen carefully and copy down what has written on the board. To make learning more successful teacher can act as a facilitator and totally student centered environment has to be build. . In the most general sense, it usually means encouraging students to use active techniques (experiments, real-world problem solving) to create more knowledge and then to reflect on and talk about what they are doing and how their understanding is changing. The teacher makes sure she understands the students' preexisting conceptions, and guides the activity to address them and then build on them.
In theoretically most of the teachers are aware of this, but how many of them actually practice this in their classrooms. If not what might be the burdens for this?