打呵欠使人在不由自主张开嘴的同时，能长长的、深深的吸一口气。普遍看法认为，人们打呵欠是由于困乏或疲倦造成的，因为他们需要氧气。

• Yawning involves opening the mouth involuntarily while taking a long, deep breath of air. It is commonly believed that people yawn as a result of drowsiness or weariness because they need oxygen.