In this clip the teacher is having a discussion with a small group about what proportionality is and allows them to use the context of constant speed to explain.
Proportionality and Constant Speed, Clip 2, 3:17
The teacher is discussing proportionality in the context of constant speed, a context that is very familiar to students. This gives them something to relate to while trying to assimilate the defintion of proportionality.
In this case when the teacher does not get a complete answer, she throws it back to the students. She also asks several students. This will help keep the students involved in their learning and create an atmosphere where students contribute and eventually challenge one anothers' answers.
Once the teacher realizes that the students are struggling, she gives them a rate to work with. This appears to help them be able to describe what is meant by constant speed.
