Common Core Math Standard: (4.OA.4) Using prior knowledge of factors and multiples, SWBAT identify prime and composite numbers from 1 – 100. Greek philosopher and mathematician, Eratosthenes, created a method for finding prime numbers using a hundreds chart. Learners will discover the history of Eratosthenes, as well as the step-by-step method of finding prime numbers up to 100. A recording sheet is given to reinforce and practice, finding the first 25 prime numbers. For extension, an optional activity is included for class or partner discussion. The lesson may be used with whole group, while the recording sheet may be completed independently or with partners.