Sorry for this very newbish question, but I read on several portal websites things like "prefabs taken from testchamber 18". So I presume that the original maps can be decompiled and opened in hammer.
Also in several custom maps, I recognized parts of portal single player, which seems to confirm it.
I wondered how you could do that, because the best way to learn for niewbies like me is often to do "intelligent stealing", so it definitely would help me.

no excuses 
