Why is negative times negative equals positive?

We have learned that the product of a positive number and a negative number is negative. How about the product of two negative numbers? How do we know that it is positive? Below are some illustrations and proof that show why is the product of two numbers positive.

Reason 1 – Pattern

Let us consider the pattern below.

$3 \times - 4 = -12$

$2 \times - 4 = -8$

$1 \times -4 = -4$

$0 \times -4 = 0$

$-1 \times - 4 = ?$

From the patterns, it should be $4$.