  Sachs and Conway-Gordon used linking number and a beautiful counting argument
to prove that every graph in the Petersen family is intrinsically linked (have
a pair of disjoint cycles that form a nonsplit link in every spatial embedding)
and thus each family member has no flat spatial embedding (an embedding for
which every cycle bounds a disk with interior disjoint from the graph). We give
an alternate proof that every Petersen family graph has no flat embedding by
applying B\"{o}hme's Lemma and the Jordan-Brouwer Separation Theorem.
