**Exercise:** *Sometimes a given rhombus can be the face of both
a "pointy" and a "flat" version of parallelepiped,
and sometimes there is only one version. What property of the rhombus determines
whether it can be used to make one or two versions of parallelepiped? Under
what condition is there only one, and then which one is it ?*

**Answer:** *The flat version has three obtuse angles meeting at
a vertex. Because the sum of the angles at any vertex can not exceed 360
degrees, the obtuse angle must be less than 120 degrees for a flat version
to exist. So, if the acute angle of the rhombus is between 60 and 90 degrees,
there are two versions, but if the acute angle is less than 60 degrees,
there is only a pointy version.*