This example demonstrates a basic tooltip interaction: hovering over a green circle reveals a text box that follows the mouse cursor. The tooltip is created as a separate HTML `div` element positioned relative to the page. The code uses `d3.select` to append an SVG circle and the tooltip div, and employs the `.on("mouseover")`, `.mousemove`, and `.mouseout` event handlers with `tooltip.style` to control visibility and position based on `event.pageX` and `event.pageY`. It runs on d3.v7 and renders all visual elements as SVG.