When I want to encourage a shy responder to address a question, I will let him/her know a few minutes in advance that a question is coming to that student. I then prep the student on hnow to answer the question. I then call on the student to promote his/her answering a question and since the student was pre-warned of the question and the answer, he/she is well-prepared to answer. This fosters more participation in coming lessons.
