Yawning

yawning
In folklore, a means by which Demons can enter the body, and the soul can escape. The custom of covering the mouth while yawning has nothing to do with politeness, but with preventing Demonic entry and soul loss. In colonial America, it was customary to snap the fingers when yawning in order to scare away Demons.

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Source:

The Encyclopedia of Demons and Demonology – Written by Rosemary Ellen Guiley – Copyright © 2009 by Visionary Living, Inc.