It sounds like a really bad horror movie, doesn't it? While this creature has an ordinary brain inside its skull, it also has a second much smaller, but still fully functioning brain squishing around inside its belly. While the skull-brain thinks, and controls the body, and goes about its brainy business, the gut-brain also thinks, and processes information, and exerts control over the body too. It would probably make a pretty cheesy horror movie, but the thing is, it's true. In fact, that creature with the extra brain is you.
We're talking about what scientists call the enteric nervous system, and all vertebrates have one, including humans. This is a collection of about a hundred million nerve cells that are wrapped around your stomach, esophagus, and intestines. While this second brain is not nearly as complex as the brain in your head, it does have its own motor and sensory neurons, and it uses many of the same brain chemicals and neurotransmitters. It even grows from the same part of the embryo that your other brain grows from.
What does this second brain do? Mostly, it coordinates the chemical and muscular activity of digestion, a process that's actually quite complicated. It also responds to messages from your other brain, especially messages about fear or danger. If you get a stomach ache or diarrhea because of stress or fear, it's this second brain that coordinates that.
So, the next time you put on your thinking cap, remember to put on a thinking sweater as well!