ELI5 Popcorn - What makes it pop?

What makes popcorn pop? Can we pop other grains like rice or quinoa? Why does the first kernel popped not burn while the others are being popped? Why do most brands of microwave popcorn require one specific side to be placed up?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: padenormous, advrider84, alex_dlc, metal_opera, uno-dos, robertsihr, diverguy1982 & cjluthy

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