Many cities are divided by a wall or a fence intended to keep warring factions separate. Often, the warring is not being done by inhabitants but by gangsters or zealots who have a vested interest in maintaining a culture of fear on their side of the barrier.
Today’s invention is a door which can be placed in any such wall at multiple locations by the government which installs it. Each door has two apertures, each with a very heavy-duty lock and also a lockable porthole.
Many keys would be quietly distributed for each door among the people living on that side.
After a period of mistrust, people may develop curiosity about the others on the other side of the divide and, without being controlled by the local mobsters, they can gradually open the porthole and take a peek. Children might begin to leave notes in the internal space and the portholes may one day be opened simultaneously, allowing a low-threat way for people to see each other.
Eventually, locals may decide that they are ready to open their door briefly. Doing so, noisily, allows the others to consider their position too. When several doors have been left open for a while the power of the local thugs on both sides is diminished and trust may be given a chance.