Items are Fun!
From the Super Mario Wiki
Items are Fun! is a secret level in World 5 of Yoshi's Island: Super Mario Advance 3. The level can either be very short or very long; the player can simply enter the GOAL! Ring right away, though they must go through the pipe if they wish to obtain a score for the level. But if they enter the GOAL! Ring right away, they cannot get 100 points (a "100%"). So the player could think that all the coins (including the red coins, and flowers) are in the sky near the GOAL! Ring if they choose to enter it right away. But really, in order to obtain a score (a "100%") they should enter the Warp Pipe near the finish. To find everything, they must enter the pipe in order to obtain a score (a "100%").
The level within the pipe contains various areas that involve the use of different items, which can range from Watermelons, Huffin' Puffin chicks, and basic Eggs, to the snowball from Ride the Ski Lifts. In order to earn 100 points, the player needs to investigate all of the areas; all the items in the game appear in this level, so the player can play around with any items they want, and must use them to maximum effect to obtain as many Red Coins and Flowers as possible.
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