This is a strange meme. I know plenty of people who were alive during the 1950s. If they are any indication, the most difficult thing to explain is how to use a modern cell phone or smart phone to make a phone call while not accidentally doing anything else with it.
(I recently got my dad what was advertised as a basic, unsmart cell phone. Every time I picked it up, I managed to trigger the voice recognition function.)
I think the other most difficult thing to explain would be certain kinds of humor that have come about since the 50s. I'm not sure how to explain which kinds, though.
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