The subject is reflexes, actions that ""you don't have to think about [to make] them happen. Reflexes happen whether you want them to or not."" For background, Berger and Keller sketch in the nervous system, then trace each of the featured reflexes from the situation that sets it off (""A bit of dust or dirt gets into your nose"") to the reaction (""The sneeze blows the dust or dirt out of your nose""). Except for hiccups (""No one knows exactly why hiccups happen,"" although ""we do know how they work""), the benefits are implicit, and underlined for good measure. It's an easily mastered lesson and an accessible side-door introduction to the nervous system, in the series tradition of manageable units.