Emily Denham, age 10, of Cape Elizabeth, Maine, for her question:

Why do earthquakes only happen in certain places?

The experts tell us that earthquakes can happen anywhere in the world. It is true that some places don't have one very often. In fact, most places in the world have not had an earthquake for hundreds of years. Then there are those other places where earthquakes happen quite often. This, we are told, is because there are weak spots in the earth's crust. This rocky crust is about 40 miles thick and you would expect it to fit around the globe in a neat shell. But actually it is cracked into several large sections, called plates.

The weak spots are along some of the cracks, where two huge crustal plates rub together. Earthquakes happen when the two plates push in opposite directions. For example, one may be inching north and its neighbor may be inching south. These movements create the shakes. Scientists are looking for signs that tell when quakes are likely to happen. Perhaps soon they will be able to give warning, so that people in the danger zone have plenty of time to escape before a big earthquake strikes.