Ted Chiang's latest collection of short stories, "Exhalation" (Knopf 2019), out now, invites the mind to stretch and bend around thorny questions of physics, consciousness, free will and storytelling.

Nitric oxide (NO) in exhaled air is believed to reflect allergic inflammation in the airways. Measured levels of exhaled NO vary with the exhaled flow rate, which therefore must be standardized. The ...