This is an exercise from a book called "Differential Topology"
2-11: Let $M$ be the sphere $x^2+y^2+z^2=1$ in 3-space. Prove that each of the Euclidean coordinates $x,y,z$ is a differentiable function on $M$.
I know how to show a function is differentiable, but I don't know what it means for a coordinate to be a differentiable function.