From the Super Mario Wiki
The Mystery Mushroom is a power-up that appears exclusively in Super Mario Maker. It resembles the sprite of a Super Mushroom from Super Mario Bros. with the question mark of a ? Block printed on its cap. When Mario touches one, he transforms into Costume Mario and takes the form of one of various costumes, many of which are based on non-Mario Nintendo characters. Many of the costumes can be unlocked by completing the 100 Mario Challenge mode on a certain difficulty or by scanning any compatible amiibo, while some are unlocked by completing an Event Course.
Upon touching one while playing, Mario takes the appearance of the costume preset to the Mystery Mushroom in editing mode. (If no specific costume for the Mystery Mushroom has been set, Mario transforms into a random character instead.) It has the same effect as an ordinary Super Mushroom as it grants Mario an extra hit point and he can break Brick Blocks, except it maintains or shrinks him down (depending on his current state) to Small Mario's size, and also allows him to crouch, which he typically cannot do in the Super Mario Bros. game style in this size.
The Mystery Mushroom is similar to the character caps in Super Mario 64 DS, which also allowed Mario to transform into other characters (and vice-versa). However, while a cap-wearing Mario kept his voice and gained the new character's abilities, Costume Mario keeps Super Mario's abilities while gaining the character's voice or sound effects in most cases.
Mystery Mushrooms have been removed from Super Mario Maker for Nintendo 3DS, although two glitches, one involving a Big Magikoopa, can make them appear in-game. When collected, they turn Mario into Small Mario, but he also has the ability to break blocks, which Small Mario normally cannot do and gives Mario another hit from enemies, as well as showing a different sprite when the player holds down on or .
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