Water brings the worms out of the earth.
X is the common thread. It is possible that independent causes are responsible for the various occurrences of the worms appearance, but the fact that they always appear after rain or heavy watering make this possibility remote. That the water and the appearance of the worms is due to some third cause is unlikely. There could be another relevant, for example, Mr. Mahlman always fertilizes before it rains and before heavy watering. Assuming that Mrs. Mahlman s observations are over a long period of time, these possibilities seem unlikely.