THE FIX: Yeast bread has a reputation for being complicated and temperamental, with a wrong step yielding flat dough. The secret is in the yeast, a live, active organism that must be kept happy. Yeast will only perform correctly in temperatures that allow it to thrive and multiply. The first step: Dissolve yeast in water that’s the correct temperature—100° to 110°. Water that is too cold will prohibit growth, and water that is too hot will kill the yeast. Use a thermometer until you feel comfortable recognizing the target temperature. Then, after the dough has been kneaded, keep it in a warm area (85°), free from drafts, for maximum yeast activity. One way to achieve this is to cover the dough and place it in a cool oven above a bowl or pan filled with boiling water. If the bread is still not rising, the yeast may have expired. To check it, dissolve one package of yeast in warm water. If the mixture produces foam, the yeast is still good. If not, it’s time to buy some more.