THE HUMAN DIGESTIVE SYSTEM is the most conventional subject matter approached so far by science writer Mary Roach—when you've written about sex, cadavers, and space travel, the alimentary canal seems downright tame. That Gulp is nonetheless bizarre, fascinating, and hilarious is a testament to Roach's personable writing style and endless curiosity. It's not so much that she chooses great subjects, as it is that she has a knack for finding what's interesting in any subject: Roach's genius lies in extending the scope of her material in surprising, creative ways.

And so we learn, in a chapter on taste, that catfish skin is covered in taste receptors, and that the fish are "basically swimming tongues"; a chapter on digestion touches on the nutritional properties of prune pits, leading to the revelation that Vancouver, Washington, once housed a pro-prune contingent known as the Prunarians. In a chapter titled "Up Theirs," Roach visits a prison and talks to a convicted murderer who's practiced in smuggling contraband in his anus. From there, it's but a brief jaunt to a court case where an inmate was "convicted of squirting a correctional officer with a feces-filled toothpaste tube, a violation of Iowa Code section 708.3B, 'inmate assault—bodily fluids or secretions.'"

By necessity, there's lots of spit, poop, and butt talk in Gulp, and those with extreme cases of squeamishness might want to sit this one out. It could be worse though! "I have tried, in my way, to exercise restraint," Roach promises. "I am aware of the website, but I did not visit."

Roach is an engaging writer, but her need to entertain can distract from the subject matter at hand—footnotes double as extended riffs, and at times they carry the air of a a slightly desperate soft-shoe.

But if the sharpest critique of a popular science writer is that she's too funny and engaging? That's really not much of a problem. Gulp is a Magic School Bus of a book, and Roach our dirty-minded, pun-loving Ms. Frizzle, guiding the reader on a digressive yet thorough journey down the alimentary canal, beginning with the relationship between smell and taste, and ending... well, you can probably figure out where it ends.