Super Mario World/Items/P-Balloon
The P-Balloon is a yellow balloon with a letter P on it. When Mario or Luigi collects a P-Balloon he swells up like a balloon, allowing him to float across large gaps that would otherwise be uncrossable. The P-Balloon has a set timer, and Mario or Luigi will start to flash when the timer is ending.
The P-Balloon also appeared occasionally in the Super Mario World television cartoon. In the episode "Mama Luigi", when Luigi gets flattened by a spiked pillar coming down from the ceiling in Bowser's Castle, Mario rolls him up like paper and restores him to normal by placing him right next to a nearby P-Balloon. (In the actual game, Bowser's Castle doesn't have any P-Balloons.)