  In 1965, E. C. Zeeman proved that the (+/-)-twist spin of any knotted sphere
in (n-1)-space is unknotted in the n-sphere. In 1991, Y. Marumoto and Y.
Nakanishi gave an alternate proof of Zeeman's theorem by using the moving
picture method. In this paper, we define a knotted 2-dimensional foam which is
a generalization of a knotted sphere and prove that a (+/-)-twist spin of a
knotted trivalent graph may be knotted. We then construct some families of
knotted graphs for which the (+/-)-twist spins are always unknotted.
