Prime. Whether a number is prime or composite depends on whether or not you can "break it down" by multiplication. Primes are the building blocks of the counting numbers in the sense that any number can be written in the form of one or more primes multiplied together. We call numbers created with two or more primes in this way <em>composite</em>, and we leave the number 1 out of the discussion as a number that's neither prime nor composite.
5only has 1 building block: 5. It <em>is </em>a building block.
