How to prove that $\mathbb{P}^1_k \times \mathbb{P}^1_k$ is a normal variety?

Question

How can I prove that $Z=\mathbb{P}^1_k \times \mathbb{P}^1_k$ is a normal variety?

I know that a variety is normal if $\mathcal{O}_{Z,x}$ is integrally closed but I don't know how to prove this in this case.