From the Super Mario Wiki
The Spring Mushroom is an item in Super Mario Galaxy and Super Mario Galaxy 2. In the former, it is first found in the Toy Time Galaxy and later reappears in the Matter Splatter Galaxy, as well as during the Beach Bowl Galaxy's Purple Coin challenge. It resembles a metallic mushroom with a spring-like top. When obtained, it wraps a spring around Mario or Luigi, turning them into Spring Mario or Spring Luigi, respectively.
The item appears again in Super Mario Galaxy 2, where it behaves the same and is exclusively found in the Chompworks Galaxy.
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