Hiccups are caused due to the involuntary contraction of diaphragm. Diaphragm is a partition that separates chest and the abdomen. It is the muscle located just below the lungs. It is caused when diaphragm becomes irritated. It is needed when we inhale air and it goes from our nose to our mouth and then from the wind pipe to your lungs. When we inhale air very quickly, air suddenly gush in and causes hiccups. When air hits your voice box, then it produces a strange sound which is called hiccup.
