Janet Klimas, age 12, of Cleveland, Ohio, for her question:

What causes waterfalls?

Every river dips down, down to the sea, but as a rule the slope is so gentle that we do not notice it. The river seems to be gliding along on level ground. But sometimes the river has to take a rig plunge on its way to the sea. It spills over such a high drop in a forming waterfall.

A river may have to leap off a cliff and splash down to a new bed far below. This is what causes some of the loveliest waterfalls in Yosemite National Park. Sometimes the bed of a river is made of hard rocks and softer rocks, Perhaps the softer rocks wear away, leaving high shelves of hard rocks in the river beds. This is what happened to the river bed under the great falls of Niagara.