ELI5 Yawning - why is hearing reduced when you yawn?

It's a myth that we yawn to intake more oxygen - so why do we yawn? Why is hearing reduced when you yawn? Why does yawning pop your ears on a plane? Why are yawns contagious?

 

... We explain like i'm five!

 

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: provin1327, torsionfree, rob3110, rhapsodysparkles, wingfoot6, fendrbud, xucker, halikan & duzzeno

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