Heather Hill, age 14 , of White Plains, N.Y., for her question:

WHEN WAS THE GREAT WALL OF CHINA BUILT?

The longest fortified line ever built in the history of the world is the Great Wall of China. Winding through northern China from Shan hai kuan on the east coast to Kansu province in north central China, much of the Great Wall was built before the Ch in period which ran from 221 to 206 B.C.

The Chinese often call it the Wall of Ten Thousand Li. The 'Ili," a Chinese measure, equals one third of a mile. The wall winds for more than 1,500 miles over mountains and across valleys. That's longer than the distance between Seattle and San Diego or Maine to Florida.

The Great Wall was built entirely by hand and actually took hundreds of years to complete. During the Sui dynasty between A.D. 589 and 618, the wall was expanded. Additional extensive work was done on it during the Ming dynasty from 1368 to 1644.