At Boing-Boing, they answer a "science question from a toddler: Why is poop brown?"
It turns out that there is a book about it.
When I was a college student, I attended a lecture by Yogi Bhajan, founder of 3HO and spiritual father of the New Mexico Sikhs, who informed us that if we were happy and holy, etc., our turds would float.
At the time, I merely wondered how Indian sages discovered that fact without a tradition of indoor plumbing.
Dr. Sheth might not agree with the guru, since he includes a chapter on "Floaters versus Sinkers" and tells Boing-Boing what I have heard elsewhere too:
Fatty poop also smells way worse than normal and it tends to float. "Like an oil slick," Dr. Sheth said.