Cause of the Seasons

Great answer here
http://www.loc.gov/rr/scitech/mysteries/seasons.html

Because the earth's axis is tilted.
It is all about the tilt of the Earth's axis. Many people believe that the temperature changes because the Earth is closer to the sun in summer and farther from the sun in winter. In fact, the Earth is farthest from the sun in July and is closest to the sun in January!
During the summer, the sun's rays hit the Earth at a steep angle. The light does not spread out as much, thus increasing the amount of energy hitting any given spot. Also, the long daylight hours allow the Earth plenty of time to reach warm temperatures.
During the winter, the sun's rays hit the Earth at a shallow angle. These rays are more spread out, which minimizes the amount of energy that hits any given spot. Also, the long nights and short days prevent the Earth from warming up. Thus, we have winter!

Related Web Sites

What causes the Seasons- From the Windows to the Universe Web site, "What causes the seasons" is explained using different learning levels (beginner, intermediate, and advanced). There are hyperlinks (bold underlined words) within the text that direct you to other related pages on the topic of seasons. “Eyes on the Sky, Feet on the Ground” - This Web site is part of The Smithsonian Astrophysical Observatory's Using Science and the Internet as Everyday Classroom tools. It contains a collection of children's online astronomy activities. You can read chapters on the Earth's rotation or orbit by going to the Table of Contents link at the bottom of the page. Earth's Seasons Equinoxes, Solstices, Perihelion, and Aphelion 1992-2020 - The United States Naval Observatory Website provides a list of the starting dates of the seasons through 2020. Also see The Seasons and the Earth's Orbit. NASA Planetary Seasons: Every planet in our solar system has seasons.

Further Reading

Asimov, Isaac. Isaac Asimov’s guide to earth and space. New York, Random House. 1991. 285 p. (Questions and Answers). Campbell, Ann-Jeanette. The New York Public Library incredible Earth: a book of answers for kids. New York, Wiley, c.1996. 186 p. Gutsch, William A. Jr. 1001 Things everyone should know about the universe. New York, Doubleday,1998. 353 p. Thompson, Luke. Earth. New York, PowerKids Press, 2001. 24 p. (Juvenile). Pasachoff, Jay. Astronomy: from the earth to the universe, 6th edition. United States, Brooks/ Cole- Thompson Learning, c. 2002. 1v. various

For more print resources...
Search on "astronomy," "earth," "seasons," "summer," "summer solstice," "weather," "winter," or "winter solstice. in the Library of Congress Online Catalog.