A single circle follows the mouse cursor across a full-screen SVG canvas. The example demonstrates React state management for interactive visualizations, using `useState` to track mouse coordinates and `useCallback` to optimize the event handler. The `onMouseMove` event on the SVG element updates the circle's `cx` and `cy` attributes in real time. Rendering is handled entirely through React's declarative JSX syntax, producing a 30-pixel-radius circle with no additional styling.