From the Super Mario Wiki, the Mario encyclopedia
The Silver Credit is a special item found in Paper Mario.
After Kooper joins Mario's party, a Koopa Troopa named Koopa Koot has favors for Mario if he wants to ask him. Koopa Koot's second favor is that he's having a hard time sleeping, so he needs a Sleepy Sheep to make him fall asleep. If Mario takes Koopa Koot's second favor and gives the Sleepy Sheep to him, he will give Mario the Silver Credit as a token of appreciation for helping him in his elderly years. He says that the Silver Credit is for playing at a place called the Playroom, a place players can gain Coins by playing one of the two minigames there. If Mario shows the Silver Credit to the Toad, the Toad will let him open up a Warp Pipe that will lead to a minigame called Jump Attack. There is another item similar to this called the Gold Credit.
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