Why do we yawn? (Does it warn us or what) and why is it contagious (so when one person yawns why does somelse then yawn)

Question: Why do we yawn? (Does it warn us or what) and why is it contagious (so when one person yawns why does somelse then yawn)
- Keywords:
  yawn
Asked by KMB to Huma, Jack, Lucy, Miranda, Peter on 16 Nov 2016.
- Lucy Oldacre-Bartley answered on 16 Nov 2016:
  
  Good question – I’m not sure anyone really knows. One hypothesis (idea) is that it is a way of cooling the brain down. In terms of why it is contagious I’m not sure I once heard that your body thinks that the other person is stealing the oxygen, I’m not sure how true this is though!
- Miranda Bane answered on 18 Nov 2016:
  
  I think we yawn to take in more oxygen. This explains why we yawn when we are sleepy becuase we need more oxygen to stay away. Yawns are contagious because when we see someone else yawn our brain automatically makes us yawn incase there is low oxygen in the air.

Question: Why do we yawn? (Does it warn us or what) and why is it contagious (so when one person yawns why does somelse then yawn)