A smiling face rendered as an SVG follows the mouse cursor across the canvas. The example combines React's `useState` and `useCallback` with D3.js to manage interactive state and coordinate updates. As the user moves the mouse, the application updates the position of the face's center, which is drawn using D3-driven SVG elements like circles for the head, eyes, and mouth. The underlying HTML hides page overflow for a full-screen, immersive interaction.