Most birds fly. Wing feathers are spread out to increase surface area and generate lift like airplane wings. Soft contour feathers overlap rigid flight feathers to make a streamlined but firm surface. Stiff tail feathers are used to steer and break. Unlike an airplane, a bird flaps its wings to generate thrust for forward movement. A typical bird body is adapted for flight. Hand bones are fused together to support the flight feathers. A downward extension from the breastbone provides a place to anchor large chest muscles that power the wings. Bird bodies are surprisingly light, thanks to thin-walled, hollow bones. For strength, the hollow channels are filled with bony struts like the scaffolding of a building. Many birds also spend a lot of time standing and walking. Their pelvic girdle is strong, supporting the bird's weight on just two hind limbs.