Students use language to present their ideas to each other, build theories together, share solution strategies, and generate definitions. By talking both to themselves and to others, students form, speak, test, and revise ideas. The give and take of regular discourse is mathematical when it focuses on communicating mathematical ideas. Look at the shape and write down anything you can tell each other. That's a rectangle.
Triangles have one, two, three sides.
One, two, three, four. A triangle has four sides. It has four sides. I know how to spell that. And it got a point on top.