Bubble

Bubbles are spherical, clear, floating objects that appear throughout the Mario series of games. They are similar in appearance to the bubbles found on Earth, except that they are much more durable, are considerably larger, and sometimes have contents within them.

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

Super Mario World
Bubbles then appeared in Super Mario World, where they can be found floating around in a few levels. Bubbles contained either power ups or Bob-ombs, Goombas, and Cheep-cheeps. Simply touching a Bubble will cause it to burst.

Super Mario World 2: Yoshi's Island
In Super Mario World 2: Yoshi's Island, 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.

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 bar. This ability of bubbles reappears in the later game, Super Mario Galaxy.

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.

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.

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.

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.

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.

''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.

New Super Mario Bros. Wii
In New Super Mario Bros Wii, if a character loses a life in Multiplayer Mode, they reappear in a bubble. The player can also go in a bubble by pressing the A button. The player is immune in the bubble and he/she can shake the Wii remote to move closer to the other players. All players who end up in a bubble, however, will act like as if they lost the level and will get sent back to the map screen.Some items are found in bubbles wich allows them to float around the level.This is very common in Free Mode.

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