Answer:
The earth goes around the sun, making a full trip once a year. It also rotates, making day and night. But it also tilts on its axis, which means that half the earth is leaned toward the sun. The United States is located in the Northern hemisphere (or half) of the earth. When the Northern half, is tilted toward the sun, it is Summer here. And when it is leaned away from the sun, it is Winter. The tilting of the earth also means that the sunlight hits our part of the earth differently, which makes the days shorter and the nights longer. Winter officially begins on the Winter Solstice, when our part of the earth is tilted farthest from the sun.

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National Geographic: Reason for the Season
http://www.nationalgeographic.com/xpeditions/activities/07/season.html
Find activities to do at home or at school to explain how the seasons change based on the tilt and rotation of the Earth and what the signs of the seasons are in your area.

World Book: Seasons
http://www2.worldbook.com/features/seasons/html/seasons.htm
At World Book's feature on the seasons, discover what causes the seasons and learn about the temperature and months for Winter in this area.