Start by choosing a point (x₀,y₀) in the domain window. This corresponds to a point (x₀,y₀,f(x₀,y₀)) on the function graph. Next choose an epsilon using the tapedeck in the control panel. The challenge is to find a small enough &delta-disc neighborhood of (x₀,y₀) in the domain so that the surface over this disc domain lies between the two cones. Use the close-up window to check whether the condition of differentiability can be met.