This example shows a simple interactive smiley face built with React and D3. A black circle follows the mouse cursor across an SVG canvas, responding to every mouse movement event. The underlying code uses React's `useState` and `useCallback` hooks to track the cursor position and re-render the circle accordingly, while the SVG element's `onMouseMove` handler captures the event data. The rendering is handled entirely through React's `ReactDOM.render` method, with D3 loaded but not actively used in the visible logic.