Keeping students accountable

One thing I've noticed in high school is that many teachers provide a warm up for students to work on, so that the teacher can get the class focused on the math topic of the day.  Once the students have been given 5 minutes to work on it, teachers will review the answers.  Some teachers work all the problems and ask if there are any questions.  Other teachers have the students provide the answers, and then ask the class if there are any questions. From what I've observed, if the teacher is going to provide the answers, the students don't usually do the problems.  They wait until the teacher writes the answers down and then copy them.  However, the teachers that choose to have students answer the questions and call on them by name will keep students accountable to the warm up.  I expect these teachers probably also have a better understanding of what content is confusing to their students.