How can I plot $f(x, y) = x^2 + y^2$ I want to plot $f(x, y) = x^2 + y^2$?
I can plot functions of a single variable but I don't know how to plot multivariable function.

In order to sketch this surface we need to consider
$$z=x^2+y^2=(\text{distance of }(x,y)\text{ from }(0,0))^2$$
So this is intuitively equivalent to the surface created by rotating the graph of $z=x^2$ about the $z$ axis.
