yawning

You yawn anytime your brain is not getting enough oxygen and it's blood that brings oxygen to your brain. So after a big meal... your body is focusing it's functioning on your stomach so there's more blood there and less in your brain. So you yawn. When you're tired your breathing gets shallow and slow, meaning less oxygen... so you yawn.

scienists do know,it definatley is known i have heard it but that was a while a go so i forgot, but i do know there is

I think yawning has something to do with oxygen deprivation. You might want to scan through the National Institute for Health website or MedLine for further explanations.

brings more oxygen to the brain

I don't think anyone (even scientists)knows why people yawn or why they are contagious.