The recurring doctrine that the hardest items live exactly where the calculator cannot reach: when an unknown constant sits in the equation (kx^2+12x+9=0, or y=mx-4 tangent to y=x^2), there is nothing to graph until the parameter is fixed, so Desmos cannot ask the question. Tangency-with-a-parameter is the archetype; the degenerate case k=0 is invisible to a graph.