Curtains? Yes, curtains.
A curtain is a piece of cloth that blocks light or drafts. In general, curtains are mostly used to minimise light in a room. It can create ambiance or atmosphere. Another common use is for privacy reasons – hiding whatever is behind the curtain. Theatres use curtains for exactly this purpose, blocking the audience’s view as things are being prepared on-stage, or at the end of a play.
Curtains (sometimes veils, though generally the same thing) are often associated with border between things – such as the veil between the living and dead in the Sword of Truth series. It usually refers to an unseen but fragile barrier between two worlds.
Simply put, curtains are made to hide things. Hiding things – akin to secrets – create friction and, when discovered, conflict. When not revealed, but known of – either by the reader or another character – it creates tension.
In short, secrets (and subsequently hiding things) are great for stories.
Thus, curtains can be used to hide things or oneself. They can be used to separate two places – also a great story, since an unseen place often attracts people (aka curiosity), and accidentally tearing it will probably have consequences.
Herewith my attempt at flash fiction (feel free to leave an opinion or a piece of flash fiction based on the subject at hand – 200 words or less in this case – of your own in the comments):
John checked that his father was still asleep. A gentle snoring arose from the sofa. He had had a long day. Perfect.
Though he knew his father was a pretty sound sleeper, John worried that the absence of his master key could somehow make him easier to rouse, so he had taken off his shoes and sneaked down the metal hall in his socks.
The key turned easily in the big metal door that his father always kept locked. When he entered, he saw the room he had seen only a few times. Tables and electronic devices. Papers and notes. But the thing that he wanted to see – the thing that had drawn him here at a risk – was the thing that was hidden behind the wine-red curtain. He wondered what it was that his father hid from him.
He stood before it for a full minute before he could gather enough courage to move again. He could hear bubbles from inside. The curtain moved smoothly against glass.
For a moment, John thought he looked in a reflection. Then he saw two more, each drifting in an enormous glass tank.