Mushroom

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The mushroom exists in five Zelda games:A Link to the Past, Link's Awakening, Ocarina of Time, Oracle of Seasons, and Majora's Mask. In A Link to the Past and Link's Awakening, the mushroom is given to the witch for magic powder. In Ocarina of Time it's merely a trading item to get some magic powder for a lost man. It is an item in the Oracle of Seasons trading sequence, a quest for the L-2 Noble Sword. In Majora's Mask, it is used to sell to one of the hag sisters for 40 rupees.

Trivia
In Link's Awakening, the Mushroom is one of many Mario references, because when Tarin (who bears an uncanny resemblance to Mario, and also shares his love of Mushrooms) eats one, he turns into a raccoon, just as Mario did in Super Mario Bros. 3.