[header] How to stop a yawn [title] Breathe through your nose and exhale through your mouth. [step] Some scientists theorize people yawn because of a lack of oxygen. Therefore, taking a few deep breaths when you feel a yawn coming on can help stop it before it starts. 
[substeps] You should breathe through your nose and out through your mouth. This allows the most oxygen to enter and channels the air into your diaphragm.