My informant told me about an old Irish superstition he knew:

“When you yawn, make the sign of the cross over your mouth to keep the devil from climbing in.”

He told me that he has done this ever since he was a little boy, and one of his uncles told him that after he yawned a devil had climbed into his throat. He also remembers his mother and father making the sign of the cross after they yawned.

I have personally never heard of this superstition until now. It shows how religion can create many customs and beliefs and how fear perpetuates them.