In this clip the teacher is going over a common difficulty he noticed from a homework problem about enlarging a photo.
Connecting Proportionality and Covariation, Clip 2, 7:47
In this case the teacher has made the decision to discuss a homework problem in class regarding photo enlargement because he noticed a large number of students making the same mistake. In this case, they are having trouble determining which quantities co-vary.
As a class they make a list of all the possible quantities as a class and discuss which are and which are not co-varying. The teacher helps when the students miss an option.
The teacher allows wait time until a student gives a response. Notice how the teacher keeps as many people as possible involved. He even asks the students to restate another student's correct respone.
They also reference the box problem to discuss the covariation between two quantities.
Notice how the teacher is putting a lot of emphasis on how the students respond to his questions about covariation. This lets the students know that it is something important.
