A smiling face made of SVG circles tracks the mouse cursor across a yellow background. The interactive visualization renders a black circular pupil that follows the pointer, while two white eye outlines and a grey-bordered yellow face remain fixed. Built with React and D3.js v6, the app uses React's `useState` and `useCallback` hooks to handle mouse movement events, updating the circle's `cx` and `cy` attributes. The SVG output is generated by React components for the background, eyes, and moving pupil.