Why water fights a Class A fire perfectly well — and actively makes a Class K kitchen fire or an energized Class C fire worse.
Fire classes aren't a bureaucratic labeling exercise — they're a categorization of what is actually burning, because different fuel types fail (and can be stopped) in fundamentally different ways. A fire in a stack of paper, a pool of gasoline, an energized breaker panel, a block of magnesium, and a deep fryer of hot oil are five different chemical and physical situations. Treat them as interchangeable and pick "whatever extinguisher is closest," and the wrong agent doesn't just fail to help — on two of these classes, it makes things measurably more dangerous.