How to make a simple Fog Effect.
Simply press the radioactive sign to open the door. when you do that, the misty steam fog stuff will come out and fill the room. Nice for horror maps, but poorly done in this example.
As you see, I really went all-out on the architecture
The sprite needs to be in valve/sprites, otherwise the map won't run.
If you don't have a folder for it, make one.
That brightness effect can be pretty interesting for more abstract maps, though I admit here it looks... silly.