Power Balloon
The P-Balloon is an item that originated in Super Mario World that appears quite rarely in both the game and in other games after it.
It has the appearance of a yellow balloon with a large white "P" printed on it (presumably meaning "Power", and possibly relating it to the P-Switch and the P-Wing). It resides inside Item Boxes and, when it is released, will float in a direction until it is picked up by Mario or Luigi.
The item's effect is to make whoever picks it up swell up like a balloon, causing them to hover in the air. They will gradually float upwards if not moved by the player, but are capable of moving in any direction for a limited amount of time. Mario or Luigi will start to flash when the item is about to wear off, and when it does they return to normal size and drop out of the air. One level in the Special World requires the player to continually pick up new P-Balloons to maintain flight while they make their way across the level.
While the P-Balloon item has been rarely seen after Super Mario World, its effects lived on in Super Mario 64 DS where, on picking up a Power Flower, Mario would blow up just as with the P-Balloon and float. The control scheme for this transformation has been changed somewhat; Mario now gradually floats downwards instead of upwards, and has to be kept in the air by tapping the jump button, similar to swimming in 2-D Mario games. Once again, he flashes when it is about to wear off and plummets when it does. Mario is the only character who is affected this way when he picks up the Power Flower.
The P-Balloon also made appearances in the Super Mario World cartoon, most notably in the episode "Mama Luigi" where Luigi uses it to float safely down a chasm and Mario uses it to re-inflate Luigi back to his normal shape after he is flattened. Here it is usually referred to simply as a "Magic Balloon".
