when you pee you release little yellow minions that go to make the bowl red. They are tiny but they are lazy and fumble slowly to the highest level on the sides of the bowls. These creatures have large bone-like claws that grow quickly and are adapted to digging. The bone comes out tinted red with iron and other red compounds. They slowly start scratching at the side of the bowl, leaving increasingly deep and stronger red every time, although this process occurs at a very slow speed. When you leave them to work for too long, the red ring forms around the side of the water and bowl. When you flush them away after leaving the pee in the toilet for them to work in for a while, they will be gone and the red ring will not increase. When you put detergent like comet in to clean it with a brush, the brush pushes the particles of detergent into the cracks, and washes away the red the more you scrub. The red ring will thus be able to be cleaned. Although this method decreases in effectiveness over time, as the scratches deepen, the large bristles in the brush will not be able to agitate the particles of detergent as far as the cracks are. This leads to older toilets looking dirtier, with larger red rings that even if cleaned, will still remain as a very slight red because of the deep parts of the scratches that were unreachable by the detergent particles will not have their small amount of red compounds washed away.
Just thought I'd share it with you guys.