Evening light comes in at the wrong angle for reading. The glass holds a ghost image of the room behind you — yourself, partially transparent, superimposed on whatever's outside.
In winter the difference between in and out shows on the glass: fog rises from the bottom edge where the cold pulls moisture from the warm air. You could write in it.
Someone has, before — there's a circle, a loop drawn in condensation by a finger that was bored or thinking, now just an outline the cold kept.
The loop fades. The cold isn't done yet. The finger that made it is elsewhere, warmed, and has no particular memory of the glass.