That is a tremendously important question! Ever since there were people, people have watched birds fly and wondered how they did it, wondering also if they too could fly. Only in the 20th century have people been able to build machines that can fly through the air (after carefully studying birds). The birds however do a better job in many ways than we do with our airplanes.
If a bird is just gliding (or "soaring"), that is, not flapping its wings, it flies in pretty much the same way that an airplane flies. The wings push air down, so by Newton's third law the air must push them up. Partly, the push comes from the angle the wings are held at and partly from the curvature of the wings. The air travels faster above the bird's wing than it does below, and this makes the pressure lower above the wing. Here's a nice page about such airfoils.
Birds are more complicated than that, and so are airplanes. Some energy source is required to overcome the unavoidable drag of pushing through the air, to lift the bird or airplane up to flying height, and to give the bird or airplane kinetic energy. Airplanes use propellers or jet engines. Birds use strong muscles in their breasts to flap their wings. In addition, bird wings are hinged, while airplane wings are riged and fixed. The bird uses its strong muscles to push its wings downwards, pushing air downwards, generating lift, and, if the wings are angled properly, also thrust. The big problem then becomes not pushing air back upwards when the bird moves its wings up for the return stroke.
This is accomplished with the hinged wings. On the downstroke, the wing is fully extended, offering its full surface area for pushing air downwards. On the upstroke, the wing folds up, presenting less area. It is a lot like rowing with oars. The oar pushes the water behind the boat on the power stroke, but must be removed from the water and, ideally, turned 90 degrees so it does not push air or water forwards on the return stroke.
Birds have a lot of adaptations for flight. Their bones are hollow and light, but strong, They have light feathers which catch the air. They can fold up their wings when they are not in use. Their lungs are extra efficient at extracting oxygen from the air (we huff and puff when running, and flying is much harder!). They eat huge amounts of high-energy food, relative to their body weight.
People go on and on about how birds fly, and it is indeed amazing that they do. For more information, why not stop by the library, or ask a teacher or professor. Since you are at the University of Illinois, you might want to talk to CJ Pennycuick here, who wrote a book and a program on bird flight performance. Here's a link to his program (it's kind of technical, and I haven't tried it out -- it may be of more interest to people studying bird migration and the ability of birds to survive long trips than to someone interested in the physics of bird flight.).
Better yet, why not go out and watch some birds flying!
(republished on 07/18/06)
Geckos have an incredible ability to stick to surfaces. Some studies suggest the over-engineered reptiles can hold hundreds of times their own body weight. In 2000, a University of California team showed that the adhesion was due to very weak intermolecular forces produced by the billions of hair-like structures, known as setae, on each gecko foot. These "Van der Waals" forces arise when unbalanced electrical charges around molecules attract one another. The cumulative attractive force of billions of setae allows geckos to scurry up walls and even hang upside down on polished glass. The reptileís grip is only released when it peels its foot off the surface.
(published on 07/26/07)