A React application renders a smiley face that follows the mouse cursor across the full browser window. The face is a single SVG circle whose center coordinates update in response to mouse movement events. The component uses React’s `useState` and `useCallback` hooks to manage the mouse position state and optimize the event handler. ReactDOM renders the SVG element, which has an `onMouseMove` listener that captures the cursor’s clientX and clientY values. The example demonstrates state-driven interaction in React without any additional data sources or external visualization libraries.