Everybody agrees that Mark Twain was funny. This is one of the very few things about which everybody agrees, which ought to make a thinking person suspicious, and this book sets out to spoil the agreement by asking a rude question: why?

Laughter looks like the most innocent thing in the world. It is not. A joke is a small machine for slipping a truth past a guard who would have stopped a sermon at the door. Twain understood this better than almost anyone before him and most who have come since. He built a career out of saying things that would have gotten a solemn man arrested, and he managed it by the simple expedient of appearing not to mean a word.

Why We Laugh follows the boy from Hannibal who became the most quoted man in America—printer's devil, riverboat pilot, failed miner, accidental journalist, reluctant prophet—and uses his life to take the clockwork of comedy apart on the workbench to see what makes it tick. Why does surprise make us laugh while repetition makes us groan? Why is a joke that lands a small triumph and a joke that misses a small public death? Why do civilizations tolerate satirists they would happily have hanged, and need them, in secret, more than their generals?

Along the way the book watches a cheerful young humorist curdle, across forty years and too many funerals, into something darker and stranger—a man who went on manufacturing laughter long after he had privately concluded that the human race was a poor joke told by an indifferent God to an empty room. Drawing on literature, psychology, history, and the noisy ongoing argument among scholars about who Twain really was, it treats him not as a monument but as a working comedian who happened to be a genius.

It is a book about laughter. It is therefore, and unavoidably, a book about the truth—which is the thing most of us can only bear to look at squarely when somebody has made us laugh first.