I've been homebrewing for over 30 years, but I'm still learning. All my education and information says you don't cover your brew kettle, in order that the bad stuff can evaporate. But it just occurred to me that every brewery I've every visited has a top on its brew kettle. Most of them are like an inverted funnel, and I assume they're feeding the evaporate up a smokestack to the outside. So what is the purpose of this lid/funnel? Does it contribute to some condensate falling back into the brew kettle? Does bad stuff fall back into the kettle also? It is to keep the brewery from becoming too hot and humid, or does it have some other purpose? Should I partially cover my brew kettle?