3.3.1 Do it by hand, first

Let's factor a number by hand first, just so that we understand the process. Let's say we want to find all the prime factors of 20.

First, we see that $20 = 2 \times 10$ , and 2 is a prime number.

Then, we see that $10 = 2 \times 5$ , and 2 is a prime number.

Lastly, we see that $5 = 5 \times 1$ , and 5 is a prime number.

At this point, the number is reduced to 1, which means there is no more factor.