Early in their collaboration, in the spring of 2002, Winston and Breazeal selected a name: Leonardo, "because this creature represents the ideal collaboration of art and science—an artist and a scientist working together to create something real," Winston said. Then, in Los Angeles, Winston went to work on Leo's body and face. One of the few guidelines from Breazeal was that Leo not look too human, lest he fall into the "uncanny valley," a concept formulated by Masahiro Mori, a Japanese roboticist. Mori tested people's emotional responses to a wide variety of robots, from non-humanoid to completely humanoid. He found that the human tendency to empathize with machines increases as the robot becomes more human. But at a certain point, when the robot becomes too human, the emotional sympathy abruptly ceases, and revulsion takes its place. People began to notice not the charmingly human characteristics of the robot but the creepy zombielike differences.

People generally relate better to animated figures that are distinctly outlandish than those that begin to approach the ideal. This is a phenomenon known to robotics researchers as "the uncanny valley"—that point where a robot is so close to lifelike yet still so short of ideal that people become focused on its imperfections.

"That's where every neuron is focused on what's wrong with the robot, on how its motion is not quite right," said Bruce Blumberg, head of the synthetic character program at the MIT Media Lab. "The uncanny valley is a very bad place to be."

Highly technical entries cover terms from computer science, electrical engineering, linguistics, mathematics, and philosophy, along with jargon like end effector (essentially the robot's hand), mechatronics (mechanics and electronics), selsyn (direction indicator), teach box (memorizes motions for a robot to duplicate), telechir (remotely controlled robot), and uncanny valley theory (that humans are more comfortable with robots that resemble humans—but only up to a point).