The Fairy Slingshot is an item found in The Legend of Zelda: Ocarina of Time. The weapon is found in the very first dungeon, Inside The Deku Tree, and is the first secondary weapon that Link obtains. The Fairy Slingshot is used to defeat or stun enemies or hit targets that Link could not hit with anything else. Each shot with this is as powerful as a slash with the Kokiri Sword, and can kill enemies in the same amount of shots as it takes slashes. Link can use this weapon only when he is a child, and when he is an adult, it is replaced with the Fairy Bow.
It is commonly used for killing Gold Skulltulas that can't be reached with the sword. It is used for killing regular Skulltulas as well, before climbing up a wall. This is very useful and sometimes necessary in almost all dungeons. In some dungeons, such as Inside Jabu-Jabu's Belly, it is needed for hitting the switches hanging on the roof to open some doors.
The ammunition used for the Fairy Slingshot are Deku Seeds, which can be found almost anywhere in the grass, bushes, or by defeating Deku Babas without cutting its stem. There are a few Business Scrubs that also sell them, but they are usually expensive and can be found for free very close.