Bert Girard, age 13, of Cumberland, Md., for his question:

WHERE ARE MOST OF THE WORLD'S LAKES?

Lakes are inland bodies of standing water. They are found at all altitudes and are located in every part of the world. Canada is where you'll find the most since that country has almost one half of all the world's lakes within its national boundaries.

Lake basins are formed by many geologic processes, such as buckling of stratified rock into large folds, displacement of large masses of rock and blocking of valleys by sediments of various kinds. The most important lake forming process, however, is glaciation, which creates lake basins by scooping out bedrock or by irregularly depositing loose rock material.

In general, lakes are more abundant in high latitudes, such as Canada, particularly in mountain regions that have been recently subjected to glacial action.

The source of lake water is rain that reaches the lake directly and by means of springs, brooks and rivers.

Lakes are always subject to destruction by drainage, filling up with sediments or evaporation of the lake water. Although rivers, springs, rain and fog replenish some of the water supply, in time every existing lake will be destroyed by the forces of nature.

In arid regions where rain is slight and evaporation great, lake levels rise and fall with the seasons and sometimes dry up completely for long periods. In lakes where evaporation prevents the water from overflowing the basin rims, substances dissolved in the water become concentrated as more water enters the basin.

The dissolved matter, brought into the lakes by tributary streams, varies in composition with the nature of the rocks in the local drainage system. The primary mineral constituent of salt lakes is common salt. Bitter lakes contain sulfates while alkali lakes contain carbonates. Borax lakes contain borates and some lakes contain combination of minerals.

Many lakes are important commercially as sources of minerals.

In addition to having important commercial values because of their minerals, some lakes are very important because of the fish that they provide. Some lakes are important as shipping arteries while others, especially some of the mountain lakes, are important as resorts.

Lakes are distinguished from bays, gulfs and seas, which are at sea level and are linked directly with the ocean.

The largest lakes of the world include the Aral and Caspian Seas and Lake Superior, that is in both the United States and Canada, and Lake Victoria in Africa. Although the Aral and Caspian Seas, located in Russian Turkestan, are called seas, they are true lakes.

The Caspian Sea is actually the world's largest lake. it covers an area of 143,243 square miles.

Lake Baikal is the deepest fresh water lake in the world and also the largest lake in Siberia. The greatest depth of the lake is 5,712 feet.