A grid of ten circles undulates vertically in SVG, their radii and positions governed by sine waves. This animation explores D3's data joins by using `d3.selectAll`, `data`, and `.join('circle')` to bind an array generated with `range` to the circles. The setInterval-driven loop updates positions every 100ms, demonstrating how enter, update, and exit selections work, with the circle count oscillating between five and fifteen.