To begin this lesson, post four to six different shapes (including various “types” of triangles in nontraditional orientations) on the board. These shapes could include circles, squares, rhombi, rectangles, hearts, and stars, among others. Have students select the triangles from among the posted shapes and note which students recognize triangles and which do not. As students identify shapes which are or are not triangles, have them explain their reasoning.
Discuss with the class what a triangle looks like as well as what its properties are. It might be helpful to draw triangles in different orientations, label their properties, and place these on a chart for future reference.
Prepare one bag of attribute blocks for each pair of children to use for sorting. Give the students colored paper or felt squares for use as workmats. (The shapes placed in the bag should include some of those posted on the board. Viewing and touching the shapes enable students to make the transition from visual to concrete experiences with the properties of triangles.)
Ask students to sort the objects found in their bag into two groups – triangles and non-triangles. Have them tell how their objects are alike and different. Move among the students, posing questions (see Questions for Students below) and listening to the conversations they have with each other about the triangles and their orientation.
An easy way to assess students’ understanding is to ask them to hold a triangle in the air. This is good practice for the song activity which follows.
To close this lesson, ask each student to select a different shape and listen to the song “Shapes” by recording artists Greg and Steve. In this song, the students are asked to stand when the shapes they are holding are named. Repeat the song until the students readily recognize their shapes.
