From the Super Mario Wiki, the Mario encyclopedia
A Metal Eggdozer is an item found in Yoshi's New Island. They are metallic and smaller versions of the Mega Eggdozer that Yoshis can use by swallowing a Metal Guy and can use them to break walls or obstacles by rolling them along the ground like a bowling ball. Unlike Mega Eggdozers, Yoshis cannot earn additional lives from the destruction a Metal Eggdozer causes. Metal Eggdozers will also reduce the Yoshi's jump height. When a Metal Eggdozer comes into contact with something unbreakable, it turns yellow and then goes the other way. If this happens one more time, it turns red and finally comes off screen after another hit. They are also essential in some boss battles (such as the battle with Bowser) and can also go underwater, where they are primarily used. One location where it is found is Slime Drop Drama.
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