Melissa Scheetz, age 12, of Columbia, Tenn., for her question:

WHAT'S THE DIFFERENCE BETWEEN A DIKE AND A LEVEE?

Dikes and levees are both wails that are made of stone and earth. They are designed to hold back water from dry land. Dikes usually protect land that would otherwise be under water most of the time. Levees protect land that is usually above water but may be flooded at times.

A levee actually is a barrier that keeps back flood waters after heavy rains have brought a river's water level up a great deal. It is a flood control device. The word "levee" comes from the French word "lever," which means "to raise."

The waters of the North Sea are kept back by a great system of dikes that make it possible for the Netherlands to have many low, fertile fields available for farming. If it were not for the dikes, the land would be flooded all the time.

The name "dike" also is given in geology to a wall of cooled molten rock between other rocks, and in engineering to a small dam that is built in addition to the main dam.