    String getWaterDrinker() {
        if (!solved) {
            solve();
        }
        for (int i = 0; i < NUM_HOUSES; i++) {
            if (houseDrink[i].equals("water")) { // NPE here if houseDrink[i] is null
                return houseNationality[i];
            }
        }
        return null;
    }
