classical mechanics - Why do we consider potential energy function $U(x)$ differentiable?

Recently when skimming through my physics-text I encountered an interesting definition of Force

$$F(x) = -\frac{\mathrm dU(x)}{\mathrm dx}$$

We were taught that some functions are continuous but not differentiable. So for the force to exist $U(x)$ has to be differentiable. So how can we prove that $U(x)$ is differentiable everywhere?