One characteristic of a whistle is that it creates a pure, or nearly pure, tone. The conversion of flow energy to sound comes from an interaction between a solid material and a fluid stream. The forces in some whistles are sufficient to set the solid material in motion. Classic examples are Aeolian tones that result in galloping power lines, or the Tacoma Narrows Bridge (the so-called "Galloping Gertie" of popular media). Other examples are circular disks set into vibration.