Oxygen

• One theory is that yawning is your body's way of giving itself more oxygen and releasing carbon dioxide. Yawning helps rid the blood of carbon dioxide while putting more oxygen into the blood.
  
  

Stretching

• Yawning helps stretch your muscles, especially the lungs and lung tissue. When your stretch your muscles your body gets ready to spring into action. Yawning helps get you ready by increasing your heart rate and helping you feel more awake.
  
  

Brain Temperature

• Another theory about why we yawn is that it helps to cool your brain. It's thought that our brains work best at a certain temperature. Yawning helps get our brains to that temperature for maximum performance.