Q: Why do recipes for Fried Rice say to use leftover rice?  Why can't I just cook the rice and use it right away?

If you were to use just-made rice, it would continue to cook during the making of the final dish.  The rice would become too soft, and starch released from it could make the entire dish mushy.  But a little kitchen science can prevent that from happening.

There are two basic types of starch molecules found in rice, as in other grains.  One type, amylose, is formed as a very long chain of glucose molecules, with few branches.  The other, amylopectin, is also made from glucose, but with lots of side branches.  In leftover rice that has been refrigerated for at least a day, the amylose molecules bond to each other and form hard crystals.  Amylopectin does the same thing, but doesn't form such hard clusters.

As the leftover rice is reheated, the starch crystals melt and the rice becomes soft again, just as the dish finishes cooking. The overall result is Fried Rice with individual grains of rice that are not over-cooked or clumped together.

Added note:  Cooked rice left at room temperature provides an ideal medium for the growth of the bacteria Bacillus Cereus, one form of which lives on uncooked rice and can survive cooking.  This bacteria can cause food poisoning.  For further details, please read the article Refrigerate Cooked and Leftover Rice from Colorado Sate University.  The bottom line is that cooked rice should be refrigerated within one hour of cooking, and foods made with leftover rice should be kept  at temperatures above 140 °F (60 °C).