Barbara Lewis, age 12, of Falmouth, Maine, for her question:

HOW DOES YEAST RISE?

Yeast is the fragrant stuff that puffs up a loaf of bread with spongy bubbles. It is a colony of single cell plants—with the know how to multiply at a fantastic rate. The warm, moist dough rises as the thriving yeast cells give off carbon dioxide as a waste product. The dough rises as this waste gas fills it with puffy bubbles. The yeast is destroyed in the hot oven. But the gassy bubbles remain to make the loaf light and digestible.