Effective questions help develop mathematical thinking in various ways. Some questions promote problem solving, some are more apt for encouraging conjectures, while others help to stimulate reflection.
What makes you say that?
While simple on the surface, this question helps students dig deeper by providing them an opportunity to reflect on their own thinking. The question is nonthreatening and communicates a sense of curiosity from the teacher. The responding student has an opportunity to share his or her interpretation and provide evidence to justify his or her reasoning. For everyone else listening, the exchange provides an opportunity to consider multiple points of view for which the validity can be judged based on the supporting evidence.
For Further Reading: Ritchhart, Ron, Mark Church, and Karin Morrison. Making Thinking Visible: How to Promote Engagement, Understanding, and Independence for All Learners. San Francisco: Jossey-Bass, 2011