Action:

• Before asking students to make a contribution to a whole class discussion, ask them to think about the question and then discuss it briefly with a neighbor.
• Consider moving throughout the classroom to listen to students’ discussion and answer any clarifying questions. This can help identify a pair that you’ll call on or have come to the board to write their thoughts and thus start the whole group discussion.
• After the discussion you can call on a specific student or pair.
• You can find more implementation details from wikipedia

Reason:

• Giving students time to silently think about their answer can give them the time to be prepared to contribute.
• Even if only a few students contribute to a whole class discussion, everyone in the class will have had the opportunity to articulate their thoughts to a peer.
• Responding as a pair can take the pressure of responsibility off a single person and make it a shared enterprise.