Here’s a strange story: One day two summers ago, Jennifer Kahn woke up because her arms — both of them — hurt. Not the way they do when you’ve slept in a funny position, but as if the tendons in her forearms and hands were moving through mud. What felt like sharp electric shocks kept sparking in her fingers and sometimes up the inside of her biceps and across her chest. Holding anything was excruciating: a cup, a toothbrush, her phone. Even doing nothing was miserable. It hurt when she sat with her hands in her lap, when she stood, when she lay flat on the bed or on her side. The slightest pressure — a bedsheet, a watch band, a bra strap — was intolerable. Our understanding of pain, and especially chronic pain, is far behind where it should be. We don’t know what causes a person with an injury to develop chronic pain, or why it happens in some people and not others, or why it happens more often in women. At a genetic and cellular level, we don’t know which systems get out of whack, or why, or how to fix them.