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. Some bubbles also replenish the air meter if Mario or Luigi is underwater.
Super Mario series
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.
Super Mario 64
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 broken after being hit with with three Vegetables.
New Super Mario Bros.
Super Mario Galaxy
In Super Mario Galaxy, Bubbles have two roles. They can be found throughout underwater areas, and will refill Mario or Luigi'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 or Luigi 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.
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 not bubble in Coin Battle mode.
The sound can be heard from the Wii Remote, that you can hear the voices from it like "Help Me" and "Hey Help", etc.
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, called Water Balls. Players can swim in them, and they also slow down Bullet Bills. They move 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 and Luigi can no longer ride around in bubbles at all, so they are used only for replenishing Mario or Luigi'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.
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. Water Balls from New Super Mario Bros. Wii appear again.
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 game.
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 Island DS
Bubbles appear throughout Yoshi's Island DS, and serve the same purpose as they did in the original Yoshi's Island.