These and many other questions are answered in the How Things Fly Web site, a companion to the physical exhibition at the Smithsonian National Air and Space Museum. The goal is to explain the basic principles of flight.
While this online exhibit is targeted towards children, the simple explanations of scientific principles, such as aerodynamics and propulsion make it accessible for persons of all ages. Many of the principles are explained by using interactive examples.
For example, the Gravity Keeps Us Down to Earth section lets you calculate how much you would weigh on another planet.
Some of the images on this site are quite striking, such as the one from the Aerodynamics section — it shows air pressure on a 747 during flight. In the image below, the colored lines indicate points of equal pressure. Red shows higher pressure, while green shows lower pressure.
There’s also an Activities section that lets you try out some of the principles of flight. Have fun!