From the Super Mario Wiki
A Slime is a gooey creature made of slime appearing in Super Mario World 2: Yoshi's Island and its remake Yoshi's Island: Super Mario Advance 3 and making a cameo in Mario Party 4. Slimes are usually a light yellow color, but in the Mario Party 4 minigame Slime Time, they are shown to come in a darker yellow, pink, purple, and green color variation.
In Yoshi's Island, they mainly appear in Tap-Tap the Red Nose's Fort and King Bowser's Castle, carrying keys. They are in disguise as blocks, until a Yoshi approaches them, which is when they turn into their true form. Like Salvo the Slime (who was actually a Lemon Drop before Kamek's transformation), they cannot directly harm Yoshi and Baby Mario, but they push them. If a Slime is hit by a seed or egg, they get smaller and smaller until they disappear. The same effect happens to the Slime if it falls into a Spike or lava pit.
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