Yeast: Fungus cells used to produce alcoholic fermentation, or to cause dough to rise . A leavening agent used in doughs and batters. It usually comes in a dry, bead like form which can be compressed into cakes, although it also exists in a fresh form. It is best activated at a temperature of 110-F to 115-F degrees. Anything too cold won't activate it, anything too hot will kill it.
Yogurt: Curdled milk which has been treated with harmless bacteria. In the Middle East it is served as a sauce with meat, fish and vegetables; in Europe mainly with or in desserts; in American primarily a breakfast or snack with fruit or fruit flavorings.