Very flexible! It comes of spending so much time sitting in contorted positions.
More properly, I'm not sure whether I can call myself mentally flexible. My ambition is to hold a "right" picture of the universe in my head, and I think I have that (roughly) done; I know more or less what things we know to exist. Getting a "right" picture of human history and society is ludicrously harder - my suspicion is that many things of that nature, such as political systems and economics, are simply beyond the capacity of the human mind, much as we'd like to think otherwise - and I recognise that I make many mistakes and must constantly reassess what I know, but still, it's not so much complete flexibility as refinement.
On a day to day level, I can deal with small amounts of change, but events I'm not expecting (say, unexpected houseguests, or being approached in conversation) stress me.
On a quirky ideas level, I'm good at thinking of unusual ways to do things, and I tend to pass things through two mental filters - one linguistic, one lyrical - which can lead to odd associations, but is somewhat constraining. Certain circumstances tend to make me mentally run over (or just plain sing) particular lyrics or say particular things (it's under my control, not a compulsion, so I don't usually do it when there's people around) which is a bit rigid.
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No one has gone missing or died.
The year is still young.