If hiccups occur due to d sudden movement of diaphragm, so what's d role of water in this. why everyone says to drink water during hiccups???????

Hiccups are caused by spasming of the diaphragm, the muscles under our lungs that controls our breathing. Just like any muscle spasm it's best controlled by slow, even stretching of the muscle. Any hiccup cure that can do that, should work. Drinking water upside down is one of the the methods that attempt to relax or stimulate the diaphragm which tend to be the most effective.
If you have chronic hiccups, drinking a glass of water upside down does not work.

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