Froggys Fog Swamp Juice is billed as “the world’s greatest fog”. According to the manufacturers, it produces a long-lasting artificial mist that has been used in haunted houses, nightclubs, skating rinks, theme parks, and even police and fire departments.

She threaded an endoscope—a tube with a light and a camera at the end—into the lizard’s nose, while allowing it to inhale the artificial smoke from a fog machine. The smoke, though harmless, contained small particles, and the camera could detect these they worked their way into the iguana’s lungs.

And to Farmer’s surprise, no matter whether the lizard breathed in or out, the smoke particles only moved in one direction.

To understand why that’s weird, consider how your own lungs work. When you