A yellow circle tracks the mouse across an SVG canvas, with its position updated on every mouse move event. The example is built with React, using `useState` and `useCallback` to manage the event handler, and leverages D3 v7's `arc` function to define the path for the smile shape. The `HappyFace` component composes the `BigCircle`, `Eye`, and `Mouth` subcomponents, each rendered as SVG elements with specified coordinates and radii.