Using Yeast

Creating The Proper Environment
Yeast feeds on sugar; flour, which contains both complex and simple carbohydrates (that is both starches and sugars), is typically an adequate food source, although sometimes sugar is added to a dough to enhance the yeast activity.
The yeast cannon interact with the food source until it is exposed to water. Yeast thrives when temperatures are moderate (75-80F is ideal). The liquid you use in a bread recipe should be at room temperatures (somewhere between 68 and 76F) for most breads; it will feel slightly cool but not cold. Warm or hot water is generally not needed unless your kitchen and the ingredients are very cold. Similarly, if the kitchen is very warm, you may want to use lsightly cooler water (60-70F) to prevent the dough from rising too quickly.
Note that many bread recipes ask you to "proof" yeast by comgining it with about twice its volume in water and letting it rest until a foam develops on the surface, a sign that the yeast is alive and working. this step is largely unnecessary with today's packaged comercial yeasts.