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Scientists have an incomplete understanding of what causes hiccups; they also do not know what purpose hiccups serve. A long list of medical disorders seems to be associated with hiccups. By far the most common are distension of the stomach and the resulting reflux of stomach acid into the esophagus. A disease or an irritation in the chest could be to blame. Hiccups may arise from a variety of neurological abnormalities, many of them involving the brain stem. Metabolic and other disorders, as well as medications that cause acid reflux, have also been linked to hiccups.
Several things happen in quick succession when a person experiences a hiccup. First the roof of the mouth lifts, as does the back of the tongue, often accompanied by a burp. Then the diaphragm and the entire set of muscles used for inhaling come together in a sudden, strong contraction. Just after that contraction begins, the vocal chords clamp shut, making the "hic" sound. The heart slows a bit. Hiccups tend to recur every few seconds, sometimes continuing for hours.