A few seats along, on the Underground, a woman, possibly in her sixties, had fallen asleep. Her head had tilted back to rest against the plastic screen that separates the benches from the standing area next to the carriage doors. These plastic screens are tough, covered in human oils from hair and skin. Graffiti is scratched into them. Scuff marks. Occasional stains.

I didn't notice the woman until someone else did. Opposite me, a younger woman, possibly in her early twenties, or even younger, late teens, possibly still a child, had pulled a sketch pad from her bag and was hastily drawing something with an ink pen. My gaze followed her gaze. She was drawing the sleeping woman, the closed eyes, the drifting head, the mouth slightly open.

The girl was trying to capture all this without the woman knowing. It was a worthy scene. The sleeping woman would never know she'd featured in it, or that she was at the centre of so much silent activity and focus. She would exist in that sketch pad until the sketch pad dissolved, and one day it would, once the girl had thrown it out, or lost it, or become famous and dead, her sketches hawked at auction, disappearing into private collections, vanishing from public view, under the ground and into the dark.

The sleeping woman's head rolled softly back and forth as the train rocked. I imagined her waking up as soon as we arrived at the next station, as soon as we became motionless. Momentum lulls us into dreams - the moment it ends, we wake. The girl would have less than a second to hide what she was doing. Her drawing would become unconscionably invasive and rude as soon as it became known, but until that point it remained harmless. The young should be left to soak up the world in any way they can.

I never actually saw the sketch. It was angled away from me and I left the train soon after. I didn't even see the woman wake. If she ever found out she was being drawn, I'll never know.