A mouse follower creates an expanding circle at the cursor position, with each circle fading out and disappearing after 2 seconds. The circles use the `d3.schemePaired` color scale to generate varied stroke colors. Built with React and D3 v5, the app tracks mouse movement via React’s `onMouseMove` event and appends SVG circles using `d3.select` with a `transition()` duration of 2000ms that animates the radius and stroke opacity.