In this activity, a rectangle turns green when its area is constant. A student explores different rectangles without any mathematical notations and comes to distinguish the constant area. Further, a grid allows quantification of the area.
Shvarts, A. (2017). Eye movements in emerging conceptual understanding of rectangle area. In Kaur, B., Ho, W.K., Toh, T.L., & Choy, B.H. (Eds.). Proceedings of the 41st Conference of the International Group for the Psychology of Mathematics Education, Vol. 1, p. 268. Singapore: PME.