Look at the characters for mountain 山, river 川, and field 田, reread the riddle, and see if you can understand why. Riddles with literary allusions are also common.
還有「一月復一月，兩月共半邊；一山又一山，三山皆倒懸。上有可耕之田，下有長流之川；六口共一室，兩口不團圓」，打的是「用」字。