From the Super Mario Wiki
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Yoob Eggs (also referred to as Eggs) are eggs in Yoob's Belly in the game Mario & Luigi: Partners in Time. Yoshis that are eaten by Yoob are put inside these eggs by RC Shroobers and placed on a conveyor. They are then put in Sunnycide's room, where he watches over them, where it is implied that they will hatch into monsters as well. These eggs are known to have a weakness to water and fire, and after Sunnycide is beaten, the Yoshis inside the eggs are freed.