In the Skyloft Marketplace, sampling wares.

Finding time to stock up on valuable merchandise, Link was browsing the Skyloft Marketplace when someone called him over. "Hey, Link, over here!" It was Bertie, the potion making husband. Link walked up to him, and found he was mixing a strange new half-translucent blue potion. "I know you traded some slime balls or whatever to Gondo for upgrades a while ago, and he traded them to me for some stamina potions. Well guess what, I found that you can use the slime to make potions! Here, I want you to try this one out for me! If I'm right, it should completely protect you from slime monsters!" Link bottled some of the potion, and sure enough the next time he fought a slime monster, out in the Faron Woods, something odd happened after he drank the potion.