Breathing from scuba is mostly a straightforward matter. Under most circumstances it differs very little from normal surface breathing. In the case of a full-face mask, the diver may usually breathe through the nose or mouth as preferred, and in the case of a mouth held demand valve, the diver will have to hold the mouthpiece between the teeth and maintain a seal around it with the lips. Over a long dive this can induce jaw fatigue, and for some people, a gag reflex. Various styles of mouthpiece are available off the shelf or as customised items, and one of them may work better if either of these problems occur.