In my classroom days, I spent a lot of time dealing with students on the diagonal properties of quadrilaterals and what they said about the quadrilateral. I wish I had dynamic geometry software back then.
Far too often my students would see or hear the word "quadrilateral" and immediately picture a familiar 4-sided figure, such as a rectangle or square. Getting them to think outside of their comfort zone was not easy. Sketches such as these two would have greatly helped. (The 2nd one was posted here first on October 13)