Cassie is nicknamed for the southern cassowary. It is a ratite. Ostriches, emus, rheas, and kiwis are also ratites. What is it about ratites that reminded engineers of their robot? 

Toughness. At 125 pounds, the cassowary is one of the world’s heaviest birds. A cornered cassowary can kick with its strong legs and cut with its claws. It can seriously injure or even kill a person! Cassie the robot won’t do that. But she is solidly built, just like a cassowary. Before Cassie, scientists created another two-legged robot called Atrias. Atrias could walk too. It moved kind of like a pogo stick, on diamond-shaped legs. But Atrias wasn’t tough like Cassie. Cassie is armored with carbon fiber and aluminum shells. Those make her more prepared to face harsh environments and tricky tasks.  

Energy. Look at an ostrich. Up under that ball of feathers, you'll find the powerful muscles that pull tendons to move those long legs. Try this. Pinch a ruler near the top. Let it hang down. Then move the top of the ruler with your other hand and see how far the bottom swings with a very small push at the top of the ruler. Like ostriches, Cassie’s hips and knees are put together to use energy efficiently. She can make big movements with just a little bit of oomph.  

No flying. Ratites can’t fly. They lack a bone in their chest that flying wings would attach to. Like ratites, Cassie MUST rely on her legs. That makes Cassie especially useful. Two-legged (bipedal) bots should be able to go wherever people can go—across rocky ground, into tight spaces, and up stairs. Those factors can make bipedal robots more useful than robots on four legs or wheels—even more useful than flying robots. Plus—a bipedal bot should be able to get up quickly after slipping. Cassie’s builders put motors into her ankles and hip joints. She can adjust her posture all the time to keep herself from falling down—just like a bird would.