The metal cap is a power-up from Super Mario 64 that can be unlocked by beating a level that has a switch that enables the metal cap boxes in specific levels to be used.

When Mario puts on the metal cap his body turns to metal and his regular walking sounds are replaced with metallic sounds. With this new ability, Mario is immune to lava but will still cry out in pain and go up in the air as if it did something, sink much faster in water and can't swim due to the added weight and will be unaffected by fall damage until the power-up wears off.