When you first open a Next.js app, the browser downloads a document: a page of HTML (HyperText Markup Language). The browser reads it, and then React makes it interactive (this step is called hydration). After that, open the Network tab of your browser's developer tools and you'll see that the app never asks for a document again. Instead you see a small, steady number of unusual requests. Some are GETs that return something that isn't HTML. Others are POSTs to the page you're already on, and their answer arrives in pieces. Those POSTs look the strangest, and they're what this post explains.