When the Metal Box is hit by an attack, it covers the character who attacked it in metal for 11.5 seconds, turning them gray and metallic in appearance. In this form, the character's gravity is increased by a huge amount, but they also have much more knockback resistance, to the point where certain attacks do not make them flinch even if they are at high damage percentages. Additionally, every voice sound the character makes gets removed, and other sound effects, such as landing sounds, get replaced by metallic sounds.
In Classic, the second-to-last opponent the player has to face is a random opponent who is permanently metal and has 1 stock. The battle takes place in Meta Crystal and is the second Mini-Boss battle in Super Smash Flash 2. On Hard difficulty or higher, the random metal character is replaced by the Metal Bros..
The Metal Box appears in Super Mario 64 as one of the three Cap Blocks containing special caps that Mario can find throughout his adventure. When starting a new game, Metal Blocks initially are outlined translucent blocks that cannot be used, and in order to make Metal Blocks solid, Mario first must find and press the Green ! Switch hidden in the Cavern of the Metal Cap. When Mario opens up a Metal Box, it will release the Metal Cap, a metallic cap that he can put on to temporarily become Metal Mario, becoming impervious to damage, capable of defeating certain enemies on contact and making him so heavy he is able to walk underwater. Its transition to SSF2 omits the metallic cap and directly turns the character into metal form when hit. Additionally, whereas in Super Mario 64 the Metal Box stays in midair so Mario can punch it from below, in SSF2 the Metal Block remains in the ground so it can be used.