From the Super Mario Wiki
A ? Coin, also known as a Question Coin, is a special Coin found in Super Mario Galaxy and Super Mario Galaxy 2. They appear in many galaxies throughout both games. When one is collected, it could cause any of the following things to appear:
A ? Coin in Super Mario Galaxy 2
Most of the time, Mario is given a time limit to collect whatever appears. If he is able to successfully collect it within the time limit, he will receive a prize. Most likely, it will be a 1-Up Mushroom, but it is also possible for it to be three 1-Up Mushrooms, a Power-up, more coins or Star Bits or, very rarely, a Rainbow Star.
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