A prime number exactly has 2 factors: 1 and itself.

Consider whether you find at least one other factor. If so, then it is not a prime number. For example, 10 is not a prime number because it can be divided by 2 and 5 (in addition to 1 and 10).

Memorize at least the first prime numbers:

2, 3, 5, 7, 11, 13, 17, 19

If you would like to have a vivid explanation with a detailed example, please read the page Prime Factorization - Mona

**Step 1: Find the factors of the number**

4 is divisible by: 1, 2, 4

**Step 2: prime number or not a prime number?**

To find out whether it is a prime number, you only need to find another factor other than 1 or the 4.

The number 4 has more factors than 1 and 4. Therefore, it's **not** a prime number!