Pluck the Piranha Flower
From the Super Mario Wiki
The easiest method of reaching the Piranha Flowers' location is to start on the tiny island. The player then needs to jump across the platforms near the start and enter the Shrinker Pipe. If the player starts on the huge island, they will have to go around the entirety of the course.
Upon jumping out of the Shrinker Pipe, or upon reaching the platform if the player went around the course, the player will then have to move around the platform to find the five Piranha Flowers. If the player is nearby, the Piranha Flower will grow out of the ground and attempt to spit fire at the player. To defeat one, the player needs to attack it once, which will cause it to shrink like a normal-sized Piranha Plant. Once all five of the Piranha Flowers are defeated, the Power Star will appear next to the Shrinker Pipe.
In the DS version, if the player is Yoshi, they need to defeat the Piranha Flowers either by eating the Mushroom found in one of the logs and walking into the Piranha Flowers or by eating and spitting out the fire they shoot back at them.
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