A circle follows the mouse cursor inside an SVG canvas. The example demonstrates React's state management and event handling by updating the circle's center coordinates on every mouse move. Built with React 16.9 and ReactDOM, it uses the `useState` hook to store the mouse position and the `onMouseMove` event to update it. The circle's `cx` and `cy` attributes are bound directly to the state, causing the SVG to re-render responsively.