Mary Stroesser, age 13, of Omaha, Neb., for her question:

WHO INVENTED CONCRETE?

Cement is a fine, gray powder that is mixed with water, sand and gravel to produce the hard, stone like material called concrete.

Ancient Romans developed cement and concrete that is very similar to the kind we use today. Some of their buildings, roads, and bridges still exist, attesting to the great durability of their product. They used lime and volcanic ash. It was then a lost art from the time of the fall of the Roman Empire in about 400 A.D. until a British engineer by the name of Hon Smeaton found the formula again in 1756.

An American named Canvass White discovered a type of rock in Madison County, N.Y. in 1818 that made natural hydraulic cement with little processing. It was used to build the Erie Canal, the first major cement construction in the United States.