Assertion: tetrahedron's base | /r/HasProperty | the plane $y=0$
Facets:
Source sentences:
text: Since the tetrahedron gets wider in the $x$ and $z$ directions as $y$ decreases, the shadow of the tetrahedron is exactly the base of the tetrahedron in the $xz$-plane (the plane $y=0$), which is the triangle pictured below.