Yo·dawg·a·hedron—placing a polyhedron inside itself, as in yo dawg, I heard you like tetrahedrons, so I put a tetrahedron in your tetrahedron so you can prove while you prove.

These are created by placing a copy of a polyhedron inside itself and then connecting the vertices of the inner object to all the vertices of the outer object (all the connections that the edges would normally make). In this way each edge from the original polyhedron is replaced with criss-crossed edges—one connecting the inner to the outer, and the other vice-versa.