Non-negative polynomials as sums of squares

Prove that any non-negative real polynomials can be expressed as a sum of squares.

More formally, if $P(x)$ is a polynomial with real coefficients such that $P(x) \geq 0 \forall x \in \mathbb{R}$, show that there exist $Q_1,Q_2,\cdots, Q_k$ polynomials with real coefficients such that $P(x) \equiv Q_1^2(x) + Q_2^2(x) + \cdots + Q_k^2(x)$.