It is remarkable how, even in 1st grade, students will immediately change an answer if a teacher questions it. It is clear from the looks on their faces that if you are asking for clarification, it must mean that they are wrong, and they scramble to come up with another response.
This is one reason that I ask students to verify, justify, or explain their answers, whether they are right or wrong (even a correct answer does not always reflect an accurate understanding). When a student gives an answer, I usually follow up with, "How do you know that?" or "Can you explain to the class how you figured that out?"
If an answer is not completely correct, an explanation helps me understand a student's thinking and address any misconceptions. It keeps me from assuming that a specific wrong answer means a specific misconception. For example, if a student were to say that 6 minus 4 is 10 rather than 2, I would assume that the student had not paid close attention to the subtraction sign. That is the most likely explanation, but there may be other reasons for the error. When the student explains, I can be certain of the mistake, and by taking more time to think it through, the student may identify the error without my guidance.
After a student gives an explanation, I encourage others to join the conversation with, "I agree (or disagree) because …." Through these conversations, students are often able to share how they arrived at an answer in a way that makes sense to their peers and presents new ideas or skills.
I began this exercise as an assessment to identify what my students needed and what they had mastered. However, like so many things I do as a teacher, I quickly found other gains.
Being wrong does not have the same stigma when the teacher does not immediately praise right answers and condemn wrong ones. My students are much less focused on right and wrong answers as they listen to one another, agree and disagree, and work their way through a question or problem.
Finally, it is often a challenge for students, especially younger students, to explain their thinking. Early in the year, I frequently hear "I just knew it" or "I knew it in my head." Pushing them to think more deeply about their processes and strategies is a struggle, but with repetition, teachers can ease students into the practice. This builds their metacognition, which makes it easier for them to repeat useful strategies or make connections between skills and ideas.
Asking "How do you know?" sounds so simple. In reality, it takes time for teachers to build and for students to become comfortable with this habit. The results, however, are well worth the investment of time. 
Jennifer Orr is a 1st grade teacher at Annandale Terrace Elementary School in Annandale, Va.
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