A prime number is a natural number greater than 1 that has only two factors, 1 and itself. In other words, a prime number is a positive integer that cannot be written as a product of two distinct integers which are greater than 1. For example, the first ten prime numbers are 2, 3, 5, 7, 11, 13, 17, 19, 23, and 29. Numbers that have more than two factors are called composite numbers. The number 1 is neither prime nor composite. Prime numbers are often seen as "building blocks" by mathematicians in number theory, and the fundamental theorem of arithmetic states that a composite number can be expressed as the product of prime numbers.