In this React 19 lesson, I take a simple counter example and use it to explain what actually happens when you call a state setter like `setCount`. On the surface, you click a button, the number goes up, and the UI updates. But under the hood React follows a process, and understanding it helps you see why React stays fast even when your app grows to hundreds of components. I break the update down into three phases: trigger, render, and commit. In the trigger phase, calling `setCount(count + 1)` doesn’t immediately update the UI — it simply tells React that this component needs to update and it gets queued. In the render phase, React reruns the component function (which is why my `console.log('SimpleCounter component rendered')` prints again) and calculates what changed by comparing the new output with the previous render. Finally, in the commit phase, React applies only the actual changes to the DOM — for example, updating just the text node inside the `h2` from 0 to 1 while leaving everything else untouched. I also demonstrate a surprising behavior: logging `count` before and after `setCount` in the same click handler shows the old value both times, even though the UI updates. That sets us up to understand state updates more deeply in the next part.