The roadside fortune teller can know your last name without talking.

In the plane rectangular coordinate system, every intersection of abscissa and ordinate corresponds to a unique point. When choosing a hard card, it is equivalent to choosing an abscissa; When you select a grid, a vertical coordinate is selected. In other words, a hard card and a square correspond to a surname. At the same time, it can also be regarded as a special arrangement. There is only one intersection between the surname on the hard card in your hand and the surname in the grid on the ground.