Why is absolute value function not a polynomial? 
Why is absolute value  function   not a polynomial?

I need  a clear  answer to this question please,?
Why couldn't we consider absolute value function as a polynomial? 

Just quoting the definition of "polynomial" does not constitute a proof. Who knows, maybe there is a certain polynomial of degree $2017$ with particular coefficients that does the job. To be serious: We have to exhibit a property of ${\rm abs}$ that no polynomial can have. In this sense Reiner Martin's answer is fine.
Here is an argument not using differentiability: If $p$ is a polynomial of degree $\geq2$ then $x\mapsto{p(x)\over x}$ is unbounded when $x\to\infty$. If $p$ has degree $1$ then $p(x)p(-x)\to-\infty$ when $x\to\infty$.
