Bubble

"If you're out of breath, grab a bubble for some air!"

- Penguin

Bubbles are floating objects that appear throughout the Mario series of games. Bubbles sometimes have contents within them, and bubbles can be ridden inside in some games. Bubbles also replenish the air meter if Mario is underwater.

Super Mario World
Bubbles first appeared in Super Mario World, where they could be found floating around in a few levels. Bubbles contained either power-ups, Bob-ombs, Galoombas, or Cheep-Cheeps. Simply touching a Bubble would cause it to burst, with whatever in it falling down.

There is another type of bubble that appear in Ghost Houses and is an enemy. It is a huge green bubble that is almost indestructible; the only way to defeat it is to perform a Slope Slide and slide to it.

Super Mario 64
Bubbles can be found in the underwater levels of Super Mario 64. They usually appear when Mario opens a Treasure Chest underwater, and contain oxygen that will refill his Health Meter. This ability of bubbles reappears in the later game, Super Mario Galaxy.

Super Mario Advance 4: Super Mario Bros. 3
The Game Boy Advance remake of Super Mario Bros. 3 has a single Bubble that can be found in the World-e level Vegetable Volley. It houses a Super Mushroom, and can only be penetrated after the player hits it with three Vegetables.

New Super Mario Bros.
Bubbles also appear in New Super Mario Bros. They are found in water levels and they carry Coins.

Super Mario Galaxy
In Super Mario Galaxy, Bubbles have two roles. They can be found throughout underwater areas, and will refill Mario's oxygen meter if he touches one, much like in Super Mario 64. There are also Bubbles that can be found on land, which Mario can enter and ride around in. The player must use the Wii Remote to aim the Bubble in the desired direction, and blow it around by pressing the A Button. In this video, it is shown that bubbles were originally going to be shaped like cubes. Water Shooters will also shoot bubbles of water on land, but these cannot be controlled and act mostly as obstacles, carrying Mario to another area or into empty space.

New Super Mario Bros. Wii
In New Super Mario Bros Wii, if a player loses a life in Multiplayer Mode, he reappears in a bubble. The player can also go in a bubble by pressing or, giving themselves immunity. The player can shake the Wii remote to move closer to the other players. However, if all players end up in a bubble, they get sent back to the map screen and lose their power-ups, but they don't lose any lives. Some items are found in bubbles which allows them to float around the level. This is very common in Free Mode. Players can't bubble in Coin Battle mode.

There are bubbles that also appear in the underwater levels and they carry coins, just like in the prequel.

Also, in World 7-2, there are water bubbles moving in mid-air that the player can jump in. Once in one, the player can swim through it to continue onward.

Super Mario Galaxy 2
Bubbles reappear in the 2010 sequel to Super Mario Galaxy, and serve the same function as they did previously, though in this game, Mario can no longer ride around in bubbles at all, so they are used only for replenishing Mario's air gauge when it gets too low. One minor difference in this game involves the Co-Star Luma being able to physically fetch air bubbles that are far away from the player, and then bring them to the player if they are about to run out of air. Water Shooters appear again and launch water bubbles just as they did in the original.

Super Mario 3D Land
In Super Mario 3D Land, bubbles appear in the background of some courses. They serve no purpose whatsoever.

New Super Mario Bros. 2
In New Super Mario Bros. 2, bubbles have the same function they did in New Super Mario Bros. Wii. They are entered by pressing a special button on the Nintendo 3DS' touch screen. A player in a bubble can approach the other by pressing. If the lead player goes too far off the other player's screen, the other will automatically enter a bubble, as opposed to dying, which would have been the case in New Super Mario Bros. Wii.

New Super Mario Bros. U
In New Super Mario Bros. U, bubbles reappear, adhering to the same rules used in New Super Mario Bros. Wii. Bubble Baby Yoshis are also capable of shooting bubbles from their mouths, which can turn most enemies into Coins, power-ups or 1-Up Mushrooms. When the player gets a power-up from a Bubble Baby Yoshi's bubble, the order in which they appear is as follows: Mushroom, Fire Flower, Ice Flower, and Super Acorn.

Super Mario 3D World
In Super Mario 3D World, players are able to put themselves inside bubbles just like in the New Super Mario Bros. series, but unlike the New Super Mario Bros. series, the last player who not inside the bubble cannot put himself or herself into the bubble. This marks the first time it can be done in a 3D Mario platformer.

Super Mario Land 2: 6 Golden Coins
Bubbles appeared in Super Mario Land 2: 6 Golden Coins. Appearing only in one level, Mario could ride around in them.

Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island and Yoshi's Island: Super Mario Advance 3, Bubbles had a number of roles. They sometimes house 1UPs, but usually appeared in the form of Morph Bubbles. If the player is hit by an enemy, Baby Mario will float away in a Bubble. Bubble Dayzees and Barney Bubbles also spit out Bubbles as their attack. These Bubbles can not hurt a Yoshi directly, but are capable of knocking him back. Yoshis can also bounce up high after jumping onto these Bubbles.

Yoshi's Story
In Yoshi's Story, Bubbles called Bubble Pops held a number of objects, usually Fruit. However, these Bubbles are fairly dense, and require an Egg in order to penetrate, although jumping atop of one repeatedly will eventually pop it.

Yoshi's Island DS
Bubbles appear throughout Yoshi's Island DS, and serve the same purpose as they did in the original Yoshi's Island.

Diddy Kong Racing
In Diddy Kong Racing, Bubble traps can be set if a player has collected three green Balloons in succession. Hitting a Bubble trap will stall whoever hit it for a fair bit of time.

Paper Mario
The Bubble Plant of Flower Fields in Paper Mario is capable of blowing Bubbles, provided Mario brings it a Bubble Berry first. Mario must then ride the bubble across a bramble patch, which he otherwise cannot cross.

Dance Dance Revolution: Mario Mix
The Big Green Bubbles from the game Super Mario World appear in Wario's Carnival Ghost House in Dance Dance Revolution: Mario Mix.

Trivia

 * In both Super Mario Galaxy and Super Mario Galaxy 2, devices called Water Shooters are often found launching bubbles of pure water that are similar in size and color to air bubbles. These bubbles do not help the player, however, and in fact often act as obstacles, as Mario can become trapped in them and be carried off into empty space.
 * In Wario Land II, a living bubble named Awabō appeared as an optional boss and it is the only living bubble shown to date.