A smiley face follows the mouse cursor across a full-screen SVG canvas. The application tracks the pointer's position and renders a yellow circle with facial features at that location, creating a playful interactive effect. Built with React and ReactDOM, the example uses the `onMouseMove` event handler and `useState` to update the circle's `cx` and `cy` coordinates dynamically. The SVG viewport matches the browser window dimensions, and the smiley face’s position updates instantly as the user moves the mouse.