Imagine a person growing wings and flapping them like a bird. That is pretend. A human's arm and chest muscles are not strong enough to flap wings big enough to lift a person. Never jump from a high place to try to fly. Birds fly by flapping their wings, and their wings are covered in feathers. Many birds have hollow bones, which keep them light so they can fly. A bird's flight muscles are big chest muscles. They pull the wings down and are anchored to a deep breastbone called the keel.

A bird's wing is shaped like an airplane wing. It makes air pressure lower above the wing than below, and that pushes the wing up. Lift is the force that holds an airplane up in the air. It is made by air moving past the wings. Without air there is no lift, so wings do not work in space. Birds with long, thin wings, such as albatrosses, can glide over the ocean for long periods. Big birds such as eagles and storks have broad wings that let them soar on rising warm air, so they save the energy of flapping.

Now let's look at two animals that are not birds, one that really flies and one that glides. A bat's wing looks like a human hand: imagine the skin between your fingers made larger, thinner and stretched. Bats have wings made of thin skin stretched between very long finger bones. Bats are the only mammals that can fly. A flying squirrel does not really fly. It glides from tree to tree. It has a loose flap of skin from its wrists to its ankles, called a patagium.

Now let's look at people who tried to build flying machines. Otto Lilienthal, a German engineer, built gliders and made more than 2,000 glider flights in the 1890s. He studied the wings of birds to design his gliders. Hang glider pilots steer by shifting their body weight, as Lilienthal did with his gliders. A hang glider is a light aircraft with cloth wings and no engine. Leonardo da Vinci had sketched flying machines with flapping wings more than 500 years ago, in the late 1400s.

Muscle power has lifted people into the air. In 1979 a pilot pedaled the Gossamer Albatross, a human-powered airplane, across the English Channel, about 36 kilometers (22 miles) in 2 hours 49 minutes. In 1988 the human-powered airplane Daedalus flew about 115 kilometers (72 miles) across the sea from Crete to Santorini in Greece. An ornithopter is a bird-like machine that flaps its wings to make lift and thrust. The Snowbird, built by a University of Toronto team, had flapping wings 32 meters (105 feet) across. In August 2010 it reportedly flew for 19.3 seconds and covered about 145 meters on its pilot's muscle power alone, after being towed up.

Words to know

• keel: deep breastbone
• glide: fly without flapping
• soar: ride warm air
• steer: guide
• reportedly: as people say