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We can square anything!

           ( -3 )^2 = 9                                 0^2 = 0

                                         5^2 = 25                      ( 3.1 )^2 = 9.61
 

All x's  ->  rule: x^2  ->  y guys
 

So, what kind of y guys come out?

Squares are never negative!

All x's  ->  rule: x^2  ->  y is greater than or equal to 0