Note that I am only referring to the 80's cartoon/toys or the Marvel comic series, NOT the recent movies which use Nanomites.
In the original G.I. Joe: Real American Hero tv show and comics, Destro has a mask made of forged Beryllium steel.
In both the show and comics, the metal mask is shaped to his head/face and moves with his mouth movements and facial expressions. Is it ever explained how this is possible?