I once painted a flat black "hole" in the floor right where everyone usually walked with the lights "on", and this caused much consternation among many people.
Painting that hole is a pretty cheap way to go..unless it becomes real and is a bottomless pit and keeps sucking up all the paint that you can pour into it!
My grandmother was very terrified and serious when she warned me about exploring the wine cellar of this house, "There's a bottomless pit in that room! Don't go in there!"
I've owned the house almost 20 years, haven't found it as of yet........
The right look, tone of voice, way of presenting yourself (hinting at owning some of them thare smartz!) bend the truth, sprinkle truth among the exagerations to stimulate their minds and scaring them becomes locked in, it's going to happen!