wpegg: Sounds frighteningly like chemistry.
"It's worse than that, it's physics Jim" - StarTrekking
QC: It's getting there. But, the door has to be made of two substances. The only way for this to be true given everything guessed so far, is that they aren't a physical substance, they would be a concept.
No, the door wasn't made of two substances, The world describing the door could also be found in the dictionary as the word used for a mixture of two substances. It doesn't mean it has to have that exact meaning. I just used it as a clue. and i gave the answer to the clue: Solution. Now its up to you to figure out what the solution is for.
BTW, has nothing to do with chemistry or physics, May have something to do with programming, building etc.