Fact of life

The pressure exerted down by fast moving air (red arrows) is less than the pressure exerted up by slow moving air (green arrows).

If you tried the paper activity from the front of this article, you might have been surprised by what happened. In most cases a person would think the paper would go down and not lift up when they blow air across the top.

It may not be what you would expect, but it is what birds and planes do to lift off the ground and fly. Blowing faster-moving air above the sheet paper lowered the air pressure above the paper. Now the air pressure below the paper is higher and creates lift. Lift does exactly what it sounds like; it lifts objects off the ground when everything is just right

Bird bodies are made to fly. They have light bones, strong legs, and specially shaped wings. Flying helps birds get away from animals that want to eat them, and makes them better hunters, too. Flying also helps them travel from cold places to warm places, called migration(my-GRAY-shun).

Let’s talk about aerodynamics (air-oh-die-NAM-icks). That’s the science of how air moves around things. It helps explain how birds fly. A bird uses its legs to push off the ground into the air. That’s called thrust. It flaps its wings to stay in the air. That’s called lift. When you put thrust and lift together, a bird flies. Its wings also have a curved shape, like an upside-down spoon. This shape helps the air move above and below the wings and keep the bird in the air. Birds’ feathers also help them fly. Air rushes through the feathers, creating more lift. 

Birds don’t always have to flap their wings to fly. Once they’re up high in the sky, they can also glide, which is kind of like getting a free ride on the air. They only have to flap their wings once in a while. They can also let a jet of air push them up even higher, to the tops of really tall trees. That’s called soaring. Different birds can fly in different ways. Sea birds can dive really fast into the ocean to catch fish. Hummingbirds can flap their wings so fast all you see is a blur. Every time you ride in an airplane, you can thank birds for showing humans how to fly.