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The most stable of structures is all around us. Try these activities to open your students' eyes.

Manmade Triangles
Have students look for evidence of triangles around them. Examples include bicycles, electrical towers, sawhorses, camera tripods, stepladders, house rooflines and some of the shapes in geodesic domes. Experiment with triangles using drinking straws connected using string inside them, drawn together and tied in knot. You can easily make structures this way to test rigidity. Or have three students hold three meter sticks together in a triangle shape, versus four students holding four meter sticks. All they have to do is move around to show the difference in stability.

Abstract Triangles
Go a step further and have students find the not-so-obvious. Examples are a book opened and stood on edge, freestanding decorative floor screens, a crooked chair where only three legs touch the floor at once, an extension ladder leaning against a house, a pencil-hand-paper combination and the imaginary "kitchen triangle" of range, refrigerator and sink.

Variations on Themes
Beyond triangles, a widely used geometric shape is the trapezoid. An example is two chair legs connected by rungs, where the distance between the legs at the seat is shorter than the distance between the two legs at the floor. Have students compare the shapes, and find the similarities between triangles and trapezoids. Also, if triangles are so stable why aren't airplane windows shaped like triangles? Will a circle always fit inside a triangle, touching all three sides?