There is a feeling you get walking down a much-plowed street in the depths of winter. It all looks smooth and solid, but the engineered surface belies many modulations, both small and not-so-small. Meaning you cannot forget vigilance of the surface. The moment you do, you find your foot not-landing as expected, in the slow topple into a large pothole. During which time you know you have made a miscalculation, but that you are already committed past the point of know-return. It is the sensation of the falling-away of the foundation of one's fundamental judgements. For a moment.

It all looks so solid. But there are always unknown somethings. Be careful not to step in them.