ELI5 Mattresses - what makes them so expensive?

What makes mattresses so expensive? What is the purpose of a box spring underneath? What is the best way and optimal position for humans to sleep? Why do we humans like to use a pillow to support our heads when we sleep?

 

... 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: musical_jinn, crackyospine, shakeanjake, epicjigsaw, nittygrittytruth and andy_clark

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