
It isn't so obvious to assemble this very simple puzzle (on the left) despite the simplicity of the two pieces.
The net of the halftetrahedron (pentahedron) is easy to draw: a square with, attached to it, two opposite equilateral triangles, and two opposite isosceles trapezoids (strips of three equilateral triangles). Now we may also decompose each pentahedron into three regular pyramids (on the right) or into two smaller pentahedra (below). So we get more tricky puzzles with six or four pieces. 

remove the 8 magenta pyramids, there is a cuboctahedron left 
remove the 24 blue pyramids, there is a stella octangula left 
one may remove the tetrahedra along the cube's edges (see the LiveGraphics3D help at the bottom of the page) 


the four yellow points may be moved to modify the parallelepiped (see LiveGraphics3D help at the bottom of the page) 
proof (Vincent Papillon): drawing on the right
The three tetrahedra build un half parallelepiped (the second half is the grey prism).
Two tetrahedra have bases with same area (the red half parallelograms) and same hight (their bases are coplanar and they have the same fourth vertex); thus they have same volume. This also holds for the two tetrahedra with green bases. The three tetrahedra have thus same volume and likewise for the three tetrahedra which build the prism. Thus the six tetrahedra (animation on the left) have same volume.
remark: This result is also the proof that the volume of a tetrahedron is the sixth of the one of the parallelepiped with which it shares three edges. On the drawing on the right the tetrahedron with the green face (where the three common edges have a common vertex) and the one with the red face have this property; on the left there are two others.
the same as above, with bright colors 
an other decomposition of the same type

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convex polyhedra  non convex polyhedra  interesting polyhedra  related subjects  November 2003 updated 20012016 