A single SVG circle follows the mouse as it moves across the browser window, cycling through a rainbow of colors every second. Built with React hooks (useState, useCallback, useEffect), the component tracks mouse coordinates via onMouseMove and updates the circle's cx, cy, and fill attributes. The color changes through a 13-color array on a one-second interval, with a CSS transition smoothing the fill change. The example demonstrates interactive SVG rendering within a React application without additional data sources.