Imagine strapping a pair of enormous wings to your arms and flapping your way into the sky. This is only an imagined possibility, because people cannot fly that way, and anyone who is curious should never jump from a high place to try it. To understand why the idea runs into such serious trouble, it helps to know what any flier has to manage before it can leave the ground. For flight, a person must overcome weight, which is gravity, and create lift to do it. A person must also make thrust for forward motion and beat drag, the resistance of the air.

Four forces act on anything that flies: lift, drag, weight and thrust, and birds and airplanes use the same four. Of these, weight is the one that a living flier carries around inside its own body. A bird needs a low body weight to fly, because as body weight goes up, the muscle output needed for flying goes up too. For that reason birds are built to be light. Many birds have hollow bones, which keep them light so they can fly. Some bird bones are hollow with struts for strength, and the jaw has a light keratin beak instead of heavy jawbones and teeth. Feathers are made of keratin as well, a tough protein, and human fingernails and hair are made of a close cousin of it.

Birds also carry a considerable amount of muscle for flapping. In birds the main flight muscles make up a bigger share of body mass than in most mammals. A bird's pectoral muscles, the chest muscles that pull the wings down, make up about 15 to 25 percent of its body weight, and they are anchored to a deep breastbone called the keel. A human's arm and chest muscles are not strong enough to flap wings big enough to lift a person. Flapping with human legs could work in theory only if the wings spanned at least 80 feet and weighed much less than the person.

A famous test of human power came in 1979, when a pilot pedaled the Gossamer Albatross, a human-powered airplane, across the English Channel. The crossing covered about 36 kilometers (22 miles) and took 2 hours 49 minutes. The airplane was 93 feet 10 inches (28.6 meters) wide and weighed 70 pounds empty. It needed less than half a horsepower (373 watts) and flew at only about 10 to 15 miles per hour. People have also repeatedly tried to build wings that flap. An ornithopter is a bird-like machine that flaps its wings to make lift and thrust. Leonardo da Vinci sketched flying machines with flapping wings more than 500 years ago, in the late 1400s. In 1893 Otto Lilienthal built a model with flapping wings like a bird's, but neither it nor its successor worked well.

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. Flapping needs more and more energy as a bird gets bigger. Small birds such as warblers can flap fast, but heavier birds, from about the size of a small hawk, rely on soaring and gliding instead. In other words, the larger the flier becomes, the more carefully it must spend its energy. Because people cannot fly like birds, people invented airplanes to fly in. The Wright brothers' first airplane flight, in 1903, lasted only 12 seconds, and on that flight the airplane flew 120 feet (37 meters) through the air.

Words to know

• thrust: forward push
• drag: air's resistance
• ornithopter: flapping machine
• pectoral: chest
• keratin: tough protein