Meta nuts are metallic hexagon nuts, similar to real-life hardware devices, which appear in industrial themedZones. They are set at either the top, middle or bottom of long and giant vertical screws which are normally attached between the ceilings and floors. These screws create long shafts, which the metal nuts can move along, creating narrow pathways going both upwards and downwards (sometimes, the screws' lower end may hang above pathways).
In gameplay, the player can stand on the metal nut and either make it move up the screw by moving to the right on it, or down by moving left on it. When a playable characters stand on a metal nut, they will stay in the same position on it as the player starts build up momentum on it, until they jump off. The metal nut's momentum will increase if there are multiple playable characters on the same metal nut, or if the player uses another speed-increasing ability on it; using the Spin Dash on the metal nut definitely makes it move faster, but the move's rolling speed will eventually decrease in some games.
Sonic the Hedgehog 2
Metal nuts first appeared in Sonic the Hedgehog 2 as one of the main gimmicks in Metropolis Zone. Sometimes, Spikes are set on the ceilings above or the floor beneath the metal nut. In addition, the metal nut only moves when the player moves it.
Many metal nut passages have Asterons along them, which will especially attack the player if the metal nuts lose momentum around them or when Sonic and Tails are running on the same metal nut. Because the metal nut makes it difficult for the playable character to avoid the Asteron's spike attack, the player is at risk of taking damage and fall down to the bottom of the metal nuts' narrow passages where Spikes may be found. Certain screw shafts for the metal nuts also have open and pointy ends above some pathways. This means that, if the player moves a metal nut past said end, it will come off the screw and fall to the floor.
If the player moves the metal nut to the highest point of the screw shaft, the playable character will get crushed against the ceiling. The metal nut can similarly crush the playable character against the floor, but only if one character stands below the metal nut while another brings it down on him.
In Wii U version of Sonic Lost World, metal nuts are only featured in a single section of Frozen FactoryZone 4. Here, the player can easily build up momentum for the metal nuts without any strenuous work as they only move at one degree of speed. Also, should the player slow down the metal nut or jump off it, the metal nut will start moving downward its screw shaft.
The section with the metal nuts has one long vertical hallway with a bottomless pit below it. Along the way, the Sprinkler will appear to drop Rings. The player has to reach the top of the hallway using several metal nuts.
By choosing the secret pathway at the mid-point of Metropolis Zone Act 2, there is a metal nut that can be moved down until it falls off the screw path. It will then fall into a lava pit, where it will function as a floating platform.
If the player falls off a metal nut in the middle of a passage, it will return to its starting point if the player leaves the area (so it is off-screen) and then return to it.
In Sonic Advance, similar types of cork gimmicks on lines arappear in Angel Island Zone, except these corks move obliquely upward by pressing right on . Otherwise, these corks will slow down if the player stops moving on them.