In the second level, there is a dragon who has a clickable brain. You get this achievement by clicking it a couple times.
Once the level starts:
1) Go back to the left screen.
2) Click on the little rock thing on the left hand side. This will make you hop up into the background.
3) Go back to the right.
4) Click the thread holding the curtain on the right side of the dragon.
5) Click the dragon's brain!