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Here's a pop quiz. What part of your body is most similar to your brain? The surprising answer is your small bowel, where most digestion occurs.

"That's the saying, you know, you've got blank for blank," Dr. Oz jokes. "But the thing about the small bowel is it has primitive messenger chemicals that tell the bowel how to work. If your bowel's not happy, those same chemicals influence your brain."

In this bowel the green stuff is bile, material in the process of being digested. Dr. Oz says it's important to listen to what your bowel tells you.

"A lot of times you don't pick up on the subtle clues," he says. "It will tell you that you feel washed out or tired or a little bit of cramping. Or, you know, if you wake up in the morning and just don't feel like yourself, you probably had something allergic that you didn't clue into."