From the Super Mario Wiki
A Courage Shell is an item found in Paper Mario: The Thousand-Year Door and Super Paper Mario. In Paper Mario: The Thousand Year Door, using one can boost Mario's or his partner's Defense by 3 points for two turns. Courage Shells can be bought in Petalburg and can also be stolen from Paratroopas. They can also be cooked up by Zess T. by cooking up a Mr. Softener and Point Swap together. Notably that it's very useful against Magnus Von Grapple 2.0's Rapid Fire Cannon.
In Super Paper Mario, it reduced damage players took by half. To make it more effective, players had to play a Minigame similar to Koops' Shell Shield. The effect can last anywhere from 25 (minimum) to 40 (maximum) seconds. In this game, if one gave a Shell Shock to Saffron the cook, she would bake a Courage Shell. However, if one gives the Courage Shell to Saffron, she will bake a Mistake. Courage Shells can be purchased from Howzit's shop in Flipside for 10 coins. Various enemies can also drop Courage Shells.
The name "Courage Shell" applied earlier to a piece of Armor available to Bowser in Super Mario RPG: Legend of the Seven Stars. In this game, the Courage Shell could be bought in Monstro Town and was one of many "shell" armors that Bowser could buy and wear.
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