From the Super Mario Wiki
Lava Drops are small, floating, fireball-like enemies found in Super Mario World 2: Yoshi's Island and its port, Yoshi's Island: Super Mario Advance 3. They move either up and down, or left and right. The player can't destroy them with regular attacks, and as a result, they have to be avoided completely. They appear in various cave levels and in groups. They can be defeated by passing through a Middle Ring, or with a ? Cloud item.
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