? Coin

A '''? Coin, also known as a Question Coin', is a special coin found in Super Mario Galaxy and Super Mario Galaxy 2''. ? Coins appear in many galaxies throughout both games. When one is collected, it could cause any of the following things to appear:
 * A trail of notes
 * A large number of Star Bits or coins
 * A power-up
 * A Sling Star
 * Another ? Coin
 * A Life Mushroom
 * A 1-Up Mushroom, sometimes three
 * A Rainbow Star

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. In the Gusty Garden Galaxy, getting all the ? Coins grants a Rainbow Star, which is the only way to break open the Gold Chomp in order to get the secret Power Star in the mission The Golden Chomp.