Younger siblings are good for one thing and one thing only: being the butt of the joke!
This guy's little sister had some friends over and they were playing with a Ouija board upstairs in another part of the house, so he decided to mess with them a bit in the simplest way possible. All he did was go to the breaker panel and quickly flip the breaker on and off in the room his sister was in. Instantly, you hear a bunch of young girls scream at the top of their lungs in horror!
Again, a simple prank, but so effective!