Semi-definite derivative with no local extremum

Question

Give an example of a function $g:\mathbb{R}^2\to\mathbb{R}$, whose derivative $Dg(x_0)$ vanishes at a point $x_0$ of $\mathbb{R}^2$. The second derivative should be semi-definite and have no local extremum in $x_0$.

I can't find one. Please help.