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 To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Join us on Twitter: https://www.twitter.com/eli5ThePodcast/ or send us an e-mail: [email protected]