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After doing the Materials Lab, have kids do this hands-on activity to compare how well different classroom building materials withstand tension, compression, and torsion.

Tug-Push-Twist-O'War

Introduce the Activity
Use a rope to demonstrate the three tests. Have two kids tug on the ends of a rope (tension), then push the ends together (compression), and finally twist the ends of the rope (torsion). With each test, have the group suggest a rating using the scale on the Activity Handout. (The rope is strong in tension but weak in compression and torsion.)

Lead the Activity

• Discuss how kids can fairly compare the different materials. Encourage them to find a consistent way of handling the materials. For example, besides pushing a material together between their hands, another way to test for compression is to place the sample on a tabletop and press down on it.
• Discuss how some materials are flexible under a type of stress (they change shape) as opposed to breaking outright. When might flexibility be desirable? When is stiffness required? (Parts of structures like the cables of suspension bridges that are built to withstand shaking loads, such as wind gusts, often have some "give." Other parts of structures, such as floor beams that support great weights, need to be rigid.)

The Big Idea
Different materials have varying abilities to withstand compression, tension, and torsion. The results may differ somewhat due to the limitations of these testing methods, but, in general, kids will find similar results to those shown below. Materials scientists study these properties of construction materials using machines that can apply enormous loads to the samples and measure their ability to withstand the stresses that result.
Strong in tension: string, yarn, pipe cleaner, popsicle stick, ceramic tile, cardboard, drinking straw, cloth, rubber band (flexible), rubber eraser, paper-towel tubes, pencil
Strong in compression: popsicle stick, clay (for a while), ceramic tile, rubber eraser, paper-towel tubes (for a while), pencil
Strong in torsion: ceramic tile, rubber eraser (for a while), paper-towel tubes, pencil