The "toy" that Arthur makes for his arts and crafts class is called a bull-roarer (sometimes it is spelled without the hyphen). The teacher wanted the students to think of something "indiany" to make, and that was his suggestion. Later in the novel when the tornadoes hit, Arthur thinks he caused the bad weather because he actually made a bull-roarer and tested it out. He tells Dan that the Hopi Indians never allow their children to play with one in the springtime because the sound can bring on death and destruction. He tells Dan that he knew about the legend, but he swung the toy anyway, and their art teacher's house was "ripped apart worse than anybody's."
A bull-roarer is a flat piece of wood attached to a string. When it is swung, it makes a whirring noise that sounds like a bull. Some Native Americans believed that this device could be used to control the weather.
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