Ring Around the Toilet

[Wade Garrett]

If you don't scrub/disinfect the toilet bowl often enough, a dark ring- bacteria, I assume- forms right around the top of the waterline.

Why there?

[Avila Kap]

Can you speak English, you retarded dupe?

[Ed Pawlowski]

Minerals in the water will accumulate there as it is the shallowest portion. Could be some bacteria also. I imagine there is some explainable action of the water action and how it is fed too.

[Scott Lurndal]

As the water in the bowl evaporates, it leaves behind minerals. Some minerals are conducive to bacterial growth.

[croy]

Right. The "evaporative edge" of the pond. The crusty build-up creates a zone of easy attachment, and ease of access to both water and air for opportunistic microbes. If there is iron coming into the bowl from your pipes, the build-up is sure to take on the color of rust (and possible kill some of the microbes).