In a windowless lab somewhere, a scientist feeds hydrocodone to fish. He begins with food—giving the fish the tools to feed themselves: a motion-sensor platform, a thin metal tube leading down into the water. When a fish swims over the platform, a green light flashes and a little snack travels down through the tube. The zebrafish mill slowly in their white tank, visiting the food platform occasionally, then drifting away to nudge at the edges of their home, to flit around like tiny bicycles coasting: quick kick, long glide.
          Then, the scientist changes the input. Instead of food, drugs come through the tube when a fish triggers the sensor. It takes them a few days to learn the trick: that swimming over the platform will make them feel good. That they’ll stop feeling pain. That their quick little bodies, instead of floating, will fly.
          Within a week, the fish crowd close around the platform, zooming and rioting, releasing more and more of the drug. The green activator light strobes above them. The opioids—like the little metallic balls that settled into holes in those handheld childhood puzzles—lock into receptors in the fish’s brains. They plug up the hurt. They send those little finned bodies sailing with euphoria.
          The scientist fidgets with the controls. He begins to raise the sensor platform higher, making the water shallow and shallower still, knowing that the fish fear shoals, knowing that they avoid danger at most any cost.
          Instead, driven frantic by craving, the fish swarm the ever-shallower platform. They get reckless, aggressive. Their bodies and their brains—seventy percent identical to ours—clamor for bliss in spite of risk. The fish cannot get enough. They buzz across the platform with their lean, striped bodies, flashing their long moon-blue fins. When the drug gets taken away from them, they grow listless, anxious.
          The scientist leans over the tank, rubbing his stubble, rumpled after weeks of this. He wonders what the next step is, wonders how to pull the fish away from the sensor, wonders what it might take to loosen the drug’s hold. He tugs at his sleeves. Maybe he has a sister—as I have a brother—whose brain, like the zebrafish’s brain, craves that bitter swallow, that floating high. Maybe, like my brother, she nurses needles between her toes, fades into some kind of glassy-eyed faraway place. Maybe she, too, cashed in on her retirement, sold her home, drew out on credit card after credit card to buy enough to fly for just a little while longer. Maybe she also dragged herself, uninsured, into a hospital one night after getting thrashed bloody in a deal gone wrong. Maybe she, too, has always been careful, tidy, kind. Maybe her art still hangs on the walls of her parent’s home. Maybe, now, she breaks things. Maybe, now, she tells her mother she hates her, twists her mother’s wrist in the driveway until she shouts with pain. Maybe her addiction is too big a place to be lost inside of. Maybe she’s unfindable, in there.
          The scientist stands up. The agitated fish have made his own heart flutter, quick and anxious as fins. He rises to go home. He rustles his answerless papers into a pile. He sets the computers to sleep, clicks off the overhead lights. He stands in the doorway for a moment, looking back. Above the fish tank, in the cavelike darkness of the lab, the scientist watches the green light clicking on, on, on.