system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: how do suction cups leave an imprint on glass that shows up again when it gets steamy, even after cleaning?
gpt
Soap joins together with minerals like calcium and magnesium in the water and sticks in the tiny pits on the glass surface that are too small to see with your eye (glass isn’t perfectly smooth).

The suction cup is there, so the shape mimics that. 

If you scrub the area with something designed to remove soap scum, like a brush with an abrasive solution, it’ll go away, because the brush and the abrasives can get in those little pits to physically remove the soap/minerals.