Third-graders use tuning forks, slide whistles, strings, and other sound-producing objects to investigate the characteristics of sound. Students learn that sound is caused by vibrations, and they explore how sound travels. They learn about the relationship of pitch and volume to the frequency and amplitude of vibrations. They discover, for example, that they can alter pitch by varying the length or tension of a string. Constructing simple stringed instruments, they discover how they can increase the volume of the sound produced by the strings. Students investigate the characteristics of another common sound-producing mechanism-the human vocal cords-and build model vocal cords. They also learn about the anatomy and functioning of the human ear. They apply what they learn in the unit by designing and building musical instruments or other sound-producing devices.