Question Coins (also known as ? Coins) are large coins that appear in Super Mario Galaxy and its sequel. They are usually found out in the open or in bonus rooms. Their appearance is a large rotating golden coin with a ? on both sides.
They cause items to appear when collected. Sometimes, they make a chain of Music Notes appear (collecting all these can yield a 1-Up Mushroom), or other times they may merely generate Star Bits. They may also be found in a chain, where collecting one Question Coin makes another one appear, and collecting the final one in the chain triggers a different item appearance. One example of this is in the Gusty Garden Galaxy, where the coins allow a Rainbow Star to spawn. This allows the player to destroy the Golden Chomp, revealing a Power Star. Most of the Question Coins have a time limit, the player has to collect all of the collectibles before time runs out.