My house attic is on the ceiling outside my sisters room, but you can't see it unless you look straight up. One day while she thought she was alone in the house, I climbed in and started kicking her door, then pulling my leg up really fast. I did that a few times, and each time she got more paranoid, until she saw my leg eventually.
The other one was printing off a freaky picture of a demented cat and sticking it to the ceiling above her bed, so she saw it when she lay down and thought there was a monster on the ceiling.
My sister is nine. I am such a rebel.