The idea is that for students to understand a system they should able to explain the role of the critical factors, or parameters in that system. To do this, they will need access to an explanation of the system, but the design also asks them to produce their own account, which means they have to pay attention to that explanation. To make sure they have understood it properly they then have to collaborate with other students to produce a joint account of the system - this is an opportunity for them to challenge other students' versions, and to be challenged, so they have to defend their thinking. Collaborating on a joint output means they must debate until they agree, which is more challenging than simply discussing. Then they have a further opportunity for feedback from other groups. Finally, each student must take responsibility for their own account, submitted to the teacher, who can give expert feedback, and see how effective their learning design has been.