If a quadratic form $Q:\ V\longrightarrow\mathbb R$ is non-vanishing except at $0$ then $Q$ is positive or negative definite

Question

Let $Q:\ V\longrightarrow\mathbb R$ be a quadratic form such that $Q(u)\ne 0$ for all $u\ne0$. Then $Q$ is positive or negative definite.

From the definition, I need to show $Q(u)>0$ (or $<0)$ for any $u\ne0$. Given $u\ne0$, without loss of generality, assuming that $Q(u)>0$. But I have no idea how to start from this, what should I do next ? Thanks.