The means by which the penis becomes erect are well known, stretching back to Leonardo da Vinci’s 15th-century anatomical drawings, and to the work of Regnier de Graaf, who “produced the most thorough investigation of the penis to date” in 1668, as Friedman writes. Soon after that, de Graaf’s student Fredrik Ruysch created wax anatomical models based on cadaver dissections. These models “showed the expanding and shrinking organ to be a marvel of hydraulic engineering,” Friedman writes, thereby helping to disprove the then-widespread theory that “air” or “wind” causes erections.

These men helped establish the modern understanding of erection physiology. Here are the basics: Chemicals in the brain signal the body to relax the corpora cavernosa, two chambers inside the penis. Blood flows in through vascular openings and becomes trapped as the body restricts outflowing veins. Meanwhile, a third chamber, the corpus spongiosum, does not inflate, keeping the urethra open so that semen can pass. Eventually, circumstances (orgasm, or maybe a sudden interruption) prompt the body to let blood flow out of the area.

In its transformed state, Kelly recognized that the penis has the hallmarks of a hydrostatic skeleton, a type of structure that maintains stiffness by means of fluid trapped in a chamber. She began to wonder why mammalian penises did not bend, while creatures made of hydrostatic skeletons, like earthworms, could coil and uncoil at will. She took on the question for her dissertation, hypothesizing that the answer lay in a 90-degree arrangement of fibers in the collagenous tissue under the skin.

Collagen, a protein abundant in the human body, is present in a layer of penile tissue called the tunica albuginea. The collagen fibers lay crimped in the flaccid penis; erection unfolds them to their full length. In her research, Kelly discovered that collagen’s molecular structure is crucial to erectile stiffness. Viewing tissue samples under a microscope, she found that, as she had suspected, they had fibers arranged at roughly 90 degrees: some running the length of the penis, and others the width. This was different from more bendable hydrostatic skeletons, whose fibers are angled like a helix.

“If the wall around the erectile tissue wasn’t there, if it wasn’t reinforced this way, the shape would change, but the inflated penis would not resist bending, and erection simply wouldn’t work,” she explained in her TED talk. (In a 2007 study in the Annals of the New York Academy of Sciences, she also wrote that “the tensile stresses … are twice as great around its circumference as the stresses along its length.” This means the fibers running width-wise expand much less than the ones running the long way, to prevent the erection from “bulg[ing] out in an aneurysm.”)

“It’s an observation with obvious medical applications in humans,” she said in her TED talk, “[and] also relevant in a broad sense, I think, to the design of prosthetics, soft robots, basically anything where change in shape and stiffness are important.”