When the Metal Box is hit by an attack, it covers the character who attacked it in metal for 11.5 seconds, turning him/her grey. In this form, the character is heavier, but also has much more resistant to knockback, to the point where certain attacks don't make him flinch if he's at low percentages. Additionally, every sound the character normally plays is replaced by metallic sounds.
For Classic, the second-to-last enemy the player has to face is a random enemy who is permanently metallic and has 1 stock. The battle takes place in Metal Cavern, and is the second Mini-Boss battle in Super Smash Flash 2. In Hard mode or higher, the random metal character is replaced by the Metal Bros..
The Metal Box first appears in Super Mario 64 as one of the three power-up caps Mario can find throughout his adventure. When starting a new game, Metal Blocks initially are outlined translucent blocks that cannot be used, so in order to make Metal Blocks solid, Mario first must find and press the Green ! Switch hidden somewhere in Peach's Castle. When Mario opens up a Metal Box, it will pop out 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, also, whereas in Super Mario 64 Metal Block stays in midair so Mario can punch it from below and sprout the cap out, in SSF2 the Metal Block lays in the ground so it can be used.