Every natural number (except 1) is either prime or composite. A **prime number** is a natural number greater than 1 that can only be divided evenly by 1 and itself. For example, 7 is a prime number because it can only be divided by the numbers 1 and 7.

A** composite number** is a natural number greater than 1 that can be evenly divided by at least one other number besides 1 and itself. For example, 6 is a composite number because it can be divided evenly by 1, 2, 3, and 6.

Composite numbers can be factored into prime factors using **factor trees**. For example, 54 factors as 2 × 27, then 27 factors as 3 × 9, and 9 factors as 3 × 3, as shown below. The prime factorization of 54 is 2 × 3 × 3 × 3.