Funny story: In 5th grade, on career day, the proprietor of an Army-Navy Surplus store explained margin to me:

I buy a shirt for $7, sell it for $10, and get $3 dollars profit and $7 to buy another shirt.

I was genuinely confused about the $10-3=$7 he was claiming he could use to buy more shirts, when it was already used to pay wholesale for the first shirt.

But maybe that was confusion about Return on Invested Capital, not confusion about margin.