Hiccups are caused by not having enough air. Scaring some one causes them to take a deep breath getting more air and stopping the hiccups. Not 100% sure that's right though.
Scaring someone can interrupt the muscle contractions that cause hiccups by resetting the diaphragm muscle. ;
Scaring someone can interrupt the cycle of involuntary diaphragm contractions that cause hiccups by prompting a sudden deep breath. This deep inhalation may help reset the diaphragm and stop the hiccups. However, this method may not work for everyone, and there are other techniques available.
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