https://youtube.com/watch?v=Yn5wdQac7Xg
There’s an old joke about the Thermos bottle that keeps things hot and cold, so someone loaded it with soup and ice cream. That joke is a little close to home when it comes to FDM 3D printers.
You want to melt plastic, of course, or things won’t print, so you need heat. But if the plastic filament gets hot too early, it will get soft, expand, and jam. Heat crawling up the hot end like this is known as heat creep and there are a variety of ways that hot ends try to cope with the need to be hot and cold at the same time. Most hotends today are air-cooled with a small fan. But water-cooled hotends have been around for a while and are showing up more and more. Is it a gimmick? Are you using, planning to use, or have used (and abandoned) water cooling on your hot end?