Rosita María Hernández has run "La Tiendita de Rosita" in Roma Norte for twenty years, and her register became an accidental classroom. Helping tourists count change, explaining prices to backpackers, and teaching customers how Mexican shopping really works became her daily routine. "Numbers don't lie," she says, organizing products on her shelves. "Twenty pesos is twenty pesos, whether you're buying pan dulce at my shop or aguacates at the tianguis."

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What makes Rosita special isn't just her grandmother patience - it's her practical knowledge of Mexican commerce. From OXXO runs to Sunday tianguis visits, she knows every shopping scenario and teaches the Spanish that actually works when you're standing at a register with people waiting behind you.

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She keeps a notebook under the counter of common mistakes - not to mock but to remember how to help the next person. Her philosophy is simple: if you can buy medicine at a farmacia when you're desperately sick, you can handle any shopping situation. Her shop officially closes at 11 PM, but everyone knows she'll open for emergencies - that's when real Spanish happens, through necessity rather than textbooks.