If you've ever seen a bicycle race, you might have noticed the riders lining up behind each other in straight columns.
Bicycle racers spend much of their energy overcoming wind resistance, so this lining up makes good sense. If you ride directly behind your opponent, she'll cut through the air resistance for you, letting you save up energy for the finish line. What's true for bicycle racers is true for other things too. Going in straight line formation is the best way to overcome wind resistance.
Which brings us to those silly geese. Like bicycle racers, geese fly in formation mainly to save energy. The formation they prefer however is shaped like the letter "V." What's going on? Are geese just plain dumb?
Not at all! Remember, geese don't only need to worry about air resistance--they need to stay aloft as well. To do this, goose wings are curved and tapered like those of an aircraft. The top surface bulges upward while the bottom stays flatter. Air rushes more quickly over the wing's bulging top, and this creates a kind of upward suction. This phenomenon is called "lift," and it's a goose's secret for staying aloft.
Lift, however, is a peculiar thing. While most of it tugs upward on the goose, some of it spills away from the wing tips as a kind of updraft. This lingering updraft spreads out behind the goose in a V-shape like the wake of a boat. If another goose puts its wings inside this updraft wake, it gets an added upward boost itself. What's the best formation to take advantage of this? It's not a straight line, it's a "V"!