Consider $\displaystyle X(n)$ the set of every ordered $\displaystyle n-$tuples of zeroes and ones. Prove that $\displaystyle X(n)$ is a metric space with the metric $\displaystyle d(x,y)$ is the number of entries where $\displaystyle x,y$ don't match.

I have no idea what to do here... I know what a metric space verifies, but I don't get how to define $\displaystyle d(x,y)$ explicitly.