Keys may open multiple doors. Even a single key can open many locks, if constructed in the appropriate way. The obverse of key is also key, at least in the function of keys to lock doors. Hence the trick of the key is its dialectic function (open/close) of access. We practice the functioning of this binary thinking (1= open, 0= closed) several times a day. Even binary thinking can become very complex in case you consider a bunch of keys or organized as a ring of keys. With a plurality of locations or rooms (home, work, leisure) the uncertainty to have the fitting key to the lock in question at your disposable becomes a bit of a puzzle. Here we are, trial and error enter the stage for most of us. Don’t worry, there are digital tools to solve the problem for you, but then the whole issue of trust comes into play more forcefully. Sharing is not only caring, but also daring. Transponders replace keys and passwords have become the new “passe partout”. This leads to the brave new world of encryption as the keyfactor for security. Encryption is key. Are you already encrypting or still turning keys?
Keys are key to understanding as well. Artists in Berlin-Schöneberg combined the portraits of persons and their bundles of keys next to each other. Keys seem to reveal key personality traits of a person. Life course researchers may investigate the “first ownership” of a key to a room, an apartment or the family home as a factor of independence, even freedom to move. Handing over the keys of a car or your home to a significant other may mark looming restrictions due to health issues.
Anyway, what is your favourite key in music? More a fan of serial music or 12-tone music? 
Science Transparent
Every year there is only one special big event (Lange Nacht der Wissenschaften) to make science more transparent to the population at large. Mostly hidden behind impressive walls and buildings guarded by porters,
science is not as transparent for the large public as it would like to be. In general people are rather intimidated or feel quickly out of place if scientist start to “explain” in lengthy formats their topics, ideas, questions and intermediate solutions. Even if the distributed open science fair is running only from 5 p.m to midnight, there are lots of things to learn and look into.
The website lists 1500+ sessions, 230 are in English language. You have to be selective or spread your interest over several years. I chose to start with, maybe, the toughest choice the Weierstrass-Institute near the Naturkundemuseum. Several projects of applied mathematics and stochastic processes were exposed and explained in more or less transparent and/or understandable form to the audiences. The talk by Julian Kern “Why chance doesn’t happen by chance” had a nice interactive format with “Kahoot-quizzes” and gave a good introduction, why it is useful to understand, statistics and stochastic processes in fields from biology, physics to social phenomena. The White board replaced already the black board and chalk.
The open doors policy, at least one day per year, raises interests and awareness for topics few people would only think that they existed already. The volume “Mathematics and Society” (Wolfgang König ed. 2016) is something I shall follow up on eventually. There are lots of applications of mathematics to social phenomena (time seen from the perspective of generations for example, image below from talk), which we have a hard time to come to grips with without the aid of mathematicians. 