1  The gastrointestinal tract is a 30-foot tube running from your mouth to your anus. Topologically, your gut has the same shape as those doughnuts you regularly pass through it.

2  The small intestine contains so many folds—down to the microscopic level—that its total surface area is about 2,700 square feet, enough to cover a tennis court.

3  The Roman physician Galen regarded the stomach as a quasi-autonomous being within us, able to “feel a lack which rouses the animal and stimulates it to seek food.”

4  Much of our basic understanding of gastric physiology comes from the work of army surgeon William Beaumont, who in 1825 observed the digestive process by inserting food into an unhealed gunshot wound in a French-Canadian trapper’s stomach.

5  The three days of Christmas: It took up to 72 hours to digest your holiday dinner. Carbs (stuffing and pumpkin pie) will be processed first. The dry, overcooked protein that is your holiday turkey came next. Fat (gravy and whipped cream) was be the last to go.

6  Maybe it’s just the company. Tryptophan, an amino acid found in turkey, is often blamed for post-meal drowsiness, but the proteins in the meat largely neutralize it.

7  Or maybe it’s those dancing sugarplums. Gorging on high-glycemic foods (lots of sugar and starch) can concentrate tryptophan in your blood plasma, boosting its effect.

8  Most of the body’s serotonin, a major mood-influencing hormone, is made not in the head but in the stomach lining.

9  The calories you burn simply digesting food account for 5 to 15 percent of your energy expenditure. Protein and alcohol require the most energy.

10  Chemistry of a cheap date: Women produce only 60 percent as much alcohol dehydrogenase, the enzyme that neutralizes booze, as men do.

11  Achalasia, a rare condition that prevents swallowing, can be treated by a shot of Botox, which relaxes the esophageal sphincter—and undoubtedly makes it look years younger.

12  Pica, an eating disorder in which sufferers develop an appetite for nonnutritive substances such as paint and dirt, affects up to 30 percent of young children. Its cause is unknown but possibly linked to subtle mineral deficiencies.

13  Your stomach’s primary digestive juice, hydrochloric acid, can dissolve metal, but plastic toys that go down the hatch will come out the other end as good as new. (A choking hazard is still a choking hazard, though.)

14  Same with crayons, hair, and chewing gum—all of which will pass through within a few days, no matter what you’ve heard.

15  You, however, are easily digestible. The pain of pancreatitis comes from fat-digesting enzymes leaking from the pancreatic duct system into surrounding tissues, literally eating you from within.

16  Water, enzymes, base salts, mucus, and bile create about two gallons of liquid that enters the large intestine. Only six tablespoons or so comes out.

17  Without the colon’s marvelous ability to recover bodily fluids, animals could not survive on dry land.

18  The loudest human burp ever recorded—107.1 decibels, about as loud as a chain saw from three feet—was produced by Londoner Paul Hunn in September 2008. On TV, no less.

19  Brown is the new green: In 2005 the Ashden Award for Sustainable Energy was given to a Rwandan prison that used the methane from human feces to fuel cooking stoves.

20  That one program saved more than $1.5 million. Think of the global implications.