It is true that the value of ss can never be less than zero because if the value of s is a negative, ss will be a postive since a negative times a negative equals a positive. If the value of s is a positive, ss will still be a positive because a positive times a positive equals a positive.
For example, S= -3
-3(-3)= 9
Another example, S= 3
3(3)=9
Therefore, it is true that the value of ss can never be less than a zero.
Hopefully, this helps!