Metal Box

The Metal Box is a green Cap Block appearing in Super Mario 64. By breaking it, Mario is able to get the Metal Cap. The switch to turn on the Metal Boxes is located in the Cavern of the Metal Cap, a secret area inside Hazy Maze Cave.

The Metal Box also appears as an item in Super Smash Bros. Melee, and returns in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS / Wii U. Much like in Super Mario 64, this item causes characters become coated in metal for a short period of time. They become heavier and fall faster, making it harder to directly KO them by blasting them away, but making it harder for them get back on the stage. Getting hit will cause the duration of the power-up to shorten. The Super Smash Bros. characters/forms Giga Bowser, Wario-Man, Mega Lucario, and Giga Mac are completely unaffected by Metal Boxes.

From Super Smash Bros. Melee onward, each game of the Super Smash Bros. series has a trophy of the Metal Box. Interestingly, in Super Smash Bros. for Wii U, the trophy states the Metal Box appears in Super Mario 64 DS, but that game replaced the colored caps with multiple characters and their Power Flowers are all found in red ? Blocks.

On a side note, the name "Metal Box" is unique to the Super Smash Bros. series; in Super Mario 64, they are generically called "green boxes".

Super Smash Bros. for Nintendo 3DS / Wii U
Blue indicates exclusive to the Wii U version.