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 be very short or very long; the player can simply enter the goal roulette (which is in the second screen) right away. However, they must go into the pipe if they wish to obtain 100 points for the level, as the rest of the level is located on the other side of it.
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 Yoshi 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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