In $\mathbb{R}^3$, let $h$ be the height function of a torus standing vertically on the top of the table. A critical point of a function is those point where the differential of the function is a zero vector. But how do I show this $h$ has four critical points? I think I should need a differential structure on torus, and then I can do the composition of $h$ and the coordinate function, thus allowing me to take partial derivative in the usual sense. But, I only know that the torus is the product manifold of $S^1$ and $S^1$. I don't know how to use that knowledge to come up with a workable local coordinate chart on this specific problem.
Any help is greatly appreciated!
P.S. Why is the formula not properly displayed on my laptop? All I see is source code. I use Chrome.