A fraction is a number we need for measuring. For counting, we have the natural and whole numbers: 1, 2, 3, 100 But when we measure something, such as a length (parts of one whole is what fractions are), it will not always be a whole number. Therefore we need numbers that are less than 1 - numbers that are the parts of 1: half of 1 (1/2), a third(1/3), a fourth(1/4), a fifth(1/5) and so on...
