There was once a farmer who paid his tithes with scrupulous care. Every year, on the appointed seasons, he set aside the priestly portion, the Levitical tenth, and the poor-tithe, and delivered each to its rightful recipient. He never shaved the weights or delayed the offering, even in lean years when every grain seemed to count against his own table.
One season, for reasons he could not fully explain, he felt moved to take half of his most productive field out of cultivation entirely. On that half he dug a wide cistern and lined it carefully with stone to catch the runoff from the winter rains. His neighbors thought he had lost his mind. He had voluntarily cut his yield in half to build a hole in the ground.
The next year, a drought fell on the region. Wells went dry. Neighbors' fields withered. The farmer's remaining half of the field, watered from his cistern, produced grain and vegetables when no one else's would. He sold at the prices a drought demands and made a fortune in a single summer (Gaster, Exempla No. 106).
The sages tell the story as a quiet teaching about maaser, the tithe. The one who gives a tenth away faithfully often finds that the Holy One whispers the right decision at the right moment, and the cistern gets dug before the rains stop coming.