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Fluid-structure interaction (FSI) occurs when a flexible or flexibly mounted structure (a rigid structure that is supported by springs) is in contact with flow, and the flow forces result in the movement of the structure. Then the movement of the structure causes the flow forces to change. This change in flow forces, in turn, results in changes in the structure’s displacement. In such systems, we cannot focus only on the structure and ignore the flow around it, or focus on the flow and ignore the structure in contact with it. The two of them must be considered together. When we study fluid mechanics, we assume that the boundaries of the fluid in contact with a structure are fixed, and we focus on what the fluid does. When we study dynamics or vibrations, we assume flow forces are negligible: A structure oscillates or moves in vacuum. In both cases, if we can no longer make these assumptions, then we have an FSI system. For example, when on a windy day a tree bends and its leaves oscillate, or when a flag flaps, or a wind chime makes music, we cannot assume that the boundaries of the flow field are rigid, or we cannot assume that there is no flow around the structure. The garden hose problem is another example of an FSI system: leave a garden hose unattended when the valve is wide open: the hose starts moving around. A hose made of steel would not have moved, neither a rubber hose with no water. When we snore, our soft palate oscillates because of the flow of air around it. We would not have snored if we did not breathe, or if we did not have a soft palate.