In this lesson, students examine and implement different strategies to find a common denominator when adding or subtracting fractions. For any pair of fractions, the product of the denominators will be a common denominator, and in many cases, the smallest choice of common denominator. In other cases, there is a smaller choice than the product, and sometimes this can be a useful choice. For example, the sum can be found, using 1,000 as a common denominator, but it also can be found, with 100 as a common denominator. Either choice will work, and there are reasons for preferring each strategy. In this lesson, students think about these ideas as they investigate different common denominators for finding sums and differences of fractions.