You need to get to the point in the game where you can get to the goat in the cage, and you have the cup you use to fill with water. Drag the cup into the place where the goat is. Then fill the cup with water and drop it down on the goat, so it splashes the goat. DO NOT bring the cup all the way up on the screen and drop it from the top of your screen. Bring the cup a little bit above the ground by the goat and then drop it. This will unlock the achievement.

If you need a visual here is my YouTube Video:
https://youtu.be/rs9mHnkDkqU

I hope this helped!