A smudge tells more. Under a slant of light, it looks less like a stain than a record of motion: loose pigment, skin oil, soot, or moisture has been dragged across a surface, leaving a thin, uneven trail where contact and force met. Its shape stores the event.That distinction matters. A stain may soak, react, or settle in place; a smudge requires lateral motion because friction and shear stress pu...More