ELI5 Aluminum Foil - should I use the shiny or dull side?

What's the difference between the shiny and dull side of aluminum foil? Are you supposed to cook with one side rather than the other? Why is it called tin foil if it's made of aluminum? Why does tinfoil affect teeth nerves so much? Why is it alumiNUM in the US and aliuminIUM in the UK?

 

... 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: genoooooo, rataridicta, originfoomanchu, bigmax, randomly_redditing, torakmclaren & odd_sonder

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