Why do we yawn?

All animals with a spine yawn.  Why do we yawn?  And is it contagious?  Research has shown that we yawn because our brain is hot and yawning helps cools the brain.  If the brain gets too warm then it doesn't operate as well and you can get exhausted and sleep deprived.  So, by yawning which increases the heart rate, the blood flow to brain, and the use of muscles in your face, this all helps cool the brain.  But why does yawning seem contagious?  Research has also shown there are links to empathy, yawning around people socially close to you, and the use of mirror neurons. Watch video. It's very interesting.