ELI5 Yawning - why is it contagious?

Why do we feel the urge to yawn just by seeing or hearing someone else do it? Do animals yawn? Why do we yawn when bored or tired? Is the oxygen explanation a myth? Why does everything sound muffled when I yawn? Why do I tear up when I yawn?

... we explain like I'm five

Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: rhapsodysparkles, stonefan, fireindeedhot, sleozz, leafmeb, greshirtbrownguy, duzzeno and fendrbud

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