  Extremely large magnetoresistance (XMR) was recently discovered in WTe$_2$,
triggering extensive research on this material regarding the XMR origin. Since
WTe$_2$ is a layered compound with metal layers sandwiched between adjacent
insulating chalcogenide layers, this material has been considered to be
electronically two-dimensional (2D). Here we report two new findings on
WTe$_2$: (1) WTe$_2$ is electronically 3D with a mass anisotropy as low as $2$,
as revealed by the 3D scaling behavior of the resistance
$R(H,\theta)=R(\varepsilon_\theta H)$ with $\varepsilon_\theta =(\cos^2 \theta
+ \gamma^{-2}\sin^2 \theta)^{1/2}$, $\theta$ being the magnetic field angle
with respect to c-axis of the crystal and $\gamma$ being the mass anisotropy;
(2) the mass anisotropy $\gamma$ varies with temperature and follows the
magnetoresistance behavior of the Fermi liquid state. Our results not only
provide a general scaling approach for the anisotropic magnetoresistance but
also are crucial for correctly understanding the electronic properties of
WTe$_2$, including the origin of the remarkable 'turn-on' behavior in the
resistance versus temperature curve, which has been widely observed in many
materials and assumed to be a metal-insulator transition.
