Koyin, at the farm on the hill in Hateno Village, needs to reach the message bottle that is floating in the middle of the pond.
Quest Location
Talk to Koyin at the farm on the hill in Hateno Village, at 3633, -2049, 0175, to start the quest.
How to Complete the Quest
The message bottle that is floating in the pond is too far away to reach with Ultrahand, so you will need to put a log into the water near the bottle, then stand on the log and use Ultrahand to move the bottle from the middle of the pond to dry land. Then use Ultrahand to place the bottle near Koyin, then talk to her.
Afterward, go into the farmhouse and talk to Koyin.
Quest Reward
Koyin gives you Hateno Cheese and opens the Hateno Cheese Shop. From now on, you can get Hateno Cheese from Koyin in exchange for Fresh Milk.