A river (blue=4) spans the middle rows of the grid. A frog (green=3) sits on the left bank, a scorpion (red=1) beside it. The frog agrees to carry the scorpion across. They move together through the river. In the masked grid, halfway across, the scorpion stings the frog — the frog's value changes from 3 to 6 (injured), and both begin to sink (move down one row). It is the scorpion's nature.