No one knows when yeast was first used to make bread,but it must have been many thousands of years ago.According to one story, the mixing the idea was quite accidental. Some yeast is said to have got into the dough by mistake causing ihe mixing to rise. Because the the loaf grew to be twice as big as normal,people though it must have happened by magic. However,as the bread tasted better than the usual flat, heavy loves,they soon used yeast in making all their bread
cakes also have holes in them made by bubbles of gas,but these are caused by a different substance. It is called Baking powder,an ingredient which has practically no flavour at all. Baking powder is a mixture of chemicals; tartaric acid and bicarbonate of soda. When a mixture of these two chemicals is exposed to moisture and heat,carbon dioxide is produced. This causes the gas to bubble through the cake mixture to make it rise.