The title sounds like a question a five-year-old would ask, and most of us would reply with something like "I don't know," or "It has to be some color." But it's a valid question, and MinuteEarth has the answers, which give us an amazing glimpse into the complex chemistry of our bodies.
The main reason we don't have the answers for our kids is that the subject of bathroom functions was once not spoken of in polite society. Or even in school. But it's part of the way our bodies work, so the more we know, the better we can take control of our own health. -via Laughing Squid