Ever sit in the aisle seat of a street car during rush hour, next to an empty seat with unidentified crud on it? You become the gatekeeper of the crud chair. Everyone wants to sit there, and you have to explain to each person why you will not let them squeeze passed you to take it. At first you will use words, later you will discover that simply pointing at the crud is explanation enough.
The crud is small and difficult to notice at first, and so it falls unto you to defend their pants, skirts, and long coats. You do so gladly, for you almost sat in the crud chair too, stopping yourself seconds before you slid over to the window seat after it was vacated. Nobody warned you, but it was close.
You wonder, sadly, about the girl who sat there earlier, and what people would think of her when they saw the crud marks on her bum that were not her doing. Were they? In all likelihood, the rest of her day was filled with the silent judgement of those who looked at her bum and saw the stain upon it. She might never even be aware of this, tossing her pants into the hamper in the dark, oblivious to her diminished esteem in the eyes of those she saw that day, or rather, of those who saw her.