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I really do not do pranks as a general rule but this one from my youth worked out rather well.
My sister lived on a corner plot of land with a paved street running alongside. From the street, which saw many trick & treaters on Halloween night, her house would be on your right, her backyard directly in front of you, and a garage with peaked roof on your left. The far side of the peaked roof was shaded by citrus and fruit trees.
I ran a monofilament line from the trees down to the base of the house, passing over the downslope of the garage’s roof and across the back yard. On this I placed two coat hangers at right angles draped with a sheet for a simple and classic ‘ghost.’ A second monofilament line allowed me to let the ghost slide down the line, matching the slope of the roof, or pull it back up toward the top.
When dusk and evening came the lines were impossible to see and as trick & treaters patrolled the neighborhood for candy, I let the ghost ‘walk’ down the garage roof.
Now, my penchant to be on that roof was well know and people simply assumed that we me in a sheet proving a little holiday spirit. As people passed, they noticed the ‘ghost’ and stopped to watch, enjoying the spirit of the performance.
Hidden on the reverse slope of the roof, I let the device slide to the very edge of the roof and for a moment it stayed there, as if I had walked to the edge.
With everyone convinced it was just me in a sheet, I let it take one more ‘step’ off the roof, hovering in the air.
People gasped and then laughed loudly as they realized the gag.
That moment, when what you think you know, a person is there in a sheet, turns into something you didn’t expect and don’t understand is the essence of both horror and comedy. A rule has been broken and it can either be shockingly funny or shockingly horrifying.