Bob is conducting a class of kids playing different musical instruments. Abby walks by and is amazed by the beautiful music. Bob offers to teach her, but first, she needs to choose an instrument. Suddenly, Grover appears carrying a small salesman suitcase filled with different instruments. He takes out a violin, and Bob explains that the violin's sounds come from the bow being pulled across the four strings, making them vibrate. He explains that vibrations make sounds and gives an example of our vocal cords vibrating when we talk. Grover asks Abby if she would like to buy the violin, but Abby would like to see more instruments first. Grover pulls out a trombone, and Bob explains that you have to make a vibration with your lips while blowing air into the trombone in order for it to make sounds. Abby is still not ready to make a decision and asks to see another instrument. As Grover rummages through his suitcase, Abby notices the two timpani drums set up in the orchestra area, and Bob suggests that she try to figure out which part of the drums vibrate. After investigating different parts of the drum, Abby finds that the top part is what vibrates. Abby gives it a try with the mallets and decides that the timpani drums are the instrument for her! Bob calls the rest of the kids back from break to work on a new song. With everyone playing an instrument, Grover feels left out, but Abby points out that he can use his voice as an instrument. Now they are all able to play and sing the new song together!