Today my dad served me coffee that tasted simultaneously too bitter and too sour.

We had used the same Aeropress; we had used the same boiled water, from the same water boiler with one setting; and we had even used the same coffee beans, from the bag I gave him for Father's Day.

I realized we had unintentionally conducted an A/B test of grinding method.¹ I had used a burr grinder while he had used a blade (spice) grinder.

A blade grinder produces coffee grounds of uneven sizes. The bigger grounds get underextracted, resulting in a sour taste. And the smaller grounds get overextracted, resulting in a bitter taste.