You cannot whistle or play any wind instrument after sundown, or you would end up summoning a ghost, because ghosts sound like whistles when they move swiftly through the air.

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This is an interesting superstition because it plays upon the fear of  ghosts, yet it also plays on contagious magic—how making a sound summons something that sounds just like it. This could be a more magical/convincing way for parents to tell children to not disturb neighbors at night.