Have you ever wondered where the holes in piece of bread come from? They are made by bubbles of gas! When making bread, four and water are mixed to from a dough. Then a small amount of yeast is added to it. Yeast is a type of fungus which grows very quickly when it is warn 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.