This example demonstrates three ways D3.js handles data updates with transitions. The top SVG updates circles without animation, the middle uses a simple transition to move circles smoothly, and the bottom shows an enter-update-exit pattern where new circles fade in, existing circles move, and removed circles fall downward and disappear. A button click regenerates random data values. The code uses D3 v7 selections, the `.join()` method, and `.transition()` with a duration of 1000ms, all rendered as SVG circles with fill opacity.