The holes in a piece of bread are made by bubbles of gas. In bread-making flour and water are mixed to form a dough. Then a small amount of yeast is added to the mixture. Yeast is type of fungus which grows very quickly when it is warm and damp. While growing, it gives off a gas which bubbles up through the dough, making it expand. It is yeast which gives bread its particular flavour and appetizing smell.
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 idea was the result of an accident. Some yeast is said to have got into the dough by chance and made it rise. Because this loaf was twice as big as normal, people thought it must be magic. But as the bread tasted better than the usual flat, heavy loaves, they soon used yeast to make all their bread.
Cakes also have holes in them made by bubbles of gas. But these are made by a different substance which leaves practically no flavour. This substance is baking-powder, which is a mixture of tartaric acid and bicarbonate of soda. When these two chemicals are mixed together, wetted and heated, they react to produce carbon dioxide. This gas bubbles through the cake mixture to make it rise while being baked.