The number 7 can be composed as the product of other numbers: for example, it is 2 × 3. That captures the idea well, but is not a good enough definition, because it has too many loopholes. Informally, primes are numbers that can’t be made by multiplying other numbers. Numbers like this are called prime numbers. To “build” 7, we must use 7! So we’re not really composing it from smaller building blocks we need it to start with. For example, he only way to build 7 by multiplying and by using only counting numbers is 7 × 1. Some numbers can’t be built from smaller pieces this way. Numbers like 10 and 36 and 49 that can be composed as products of smaller counting numbers are called composite numbers. Prime and composite numbers: We can build 36 from 9 and 4 by multiplying or we can build it from 6 and 6 or from 18 and 2 or even by multiplying 2 × 2 × 3 × 3. But only some counting numbers can be composed by multiplying two or more smaller counting numbers. Building numbers from smaller building blocks: Any counting number, other than 1, can be built by adding two or more smaller counting numbers.
