Sketch $\displaystyle \sqrt{13 - x^2 - y^2}$

Attempt:

$\displaystyle z^2 = 13 - x^2 - y^2$

$\displaystyle z^2 = -x^2 - y^2 + 13$

$\displaystyle x^2 + y^2 + z^2 = 13$

Now I dont understand why this is half of a sphere (which opens up downwards) and not a whole sphere?