Investigate what quadrilaterals can be made by dragging the vertices on the given line. Can ABCD be a square, rectangle, rhombus, trapezium, kite, ...? You can change the angle between the yellow lines. Would that affect what kind of quadrilaterals you can make?
Reference: "Developing geometrical reasoning in the secondary school: outcome of trialling teaching activities in classrooms" (pp.17-32 & 77-78), downloaded from http://eprints.soton.ac.uk/9801/ .
Created with C.a.R. by R. Grothmann