Even numbers are multiples of 2, and more generally, multiples arise throughout mathematics and everyday life. Students first meet the distinction between odd numbers and even numbers in early primary school, but it is useful everywhere in mathematics. The only numbers in the module are whole numbers, apart from the final paragraphs, where fractions are used so that the fifth index law can be presented in a more satisfactory form. The ideas of this module are presented in purely arithmetical form, and no algebra is used except in some remarks that look forward to later work. This module encourages multiplicative thinking about numbers, and introduces ideas that are essential skills in fractions and algebra. Multiplication and division of whole numbers throw up many surprising things.