Why should f(x,y), g(x,y) be called tangent at (0, 0)?

[petrol.veem]

Why should the graphs of f(x,y) = x^2 + y^2 and g(x,y) = -x^2 - y^2 + x(y^3) be called tangent at (0, 0)?

I am having problems understand this question. I know how to find tangent planes, partial derivatives, etc, but i can't seem to be able to apply any of it to this question to show why they should be called tangent at the origin.

any ideas or explainations?