Remember the fun you had as a kid walking through the freshly fallen leaves, listening to them crunch under your feet, smelling that fragrance of lost summer, eying the array of color, sometimes all in one leaf? Maybe like me you had the pleasure of jumping into the leaf piles your dad had just spent hours raking up, burying your sibling or getting covered yourself, bits of leaves forcing their way down your shirt, tickling your neck. And perhaps you too collected the best and the brightest of those leaves to seal between sheets of wax paper to hang in a sunny window like a piece of stained glass.
Leaves don't really change color; the colors are always there, but the large amount of chlorophyll (this is what converts the sun's energy into food for the tree) that gives the leaves their green color through the growing season masks the other colors. When the chlorophyll dissipates in the fall, the other colors show through. The colder weather also causes the trapped glucose in some leaves to produce bright reds and purples. The shades of brown are from trapped waste.
Trees drop their leaves in the fall to keep from dying of thirst because leaves use up huge amounts of water. During most of the year that's not an issue, but in the winter, "psysiological" drought occurs and the tree can't absorb enough water. It mostly survives on stored food and water in its trunk and branches. Without its leaves, the tree can survive for a much longer time making it through until spring arrives, thawing the ground and bringing quenching rains.
"Autumn is a second Spring, when every leaf is a flower."
- Albert Camus