Hiccups are caused by a SPASM of the diaphragm, which is the muscle that separates the chest and abdominal cavities. This SPASM causes a sudden inhalation of air, which is then followed by the closure of the vocal cords, resulting in the characteristic 'hic' sound. The cause of the SPASM is not always known, but can be triggered by eating or drinking too quickly, excitement, or even a sudden change in temperature.