Arahants are like fertile fields. Givers are like the farmers, and what they offer is the seed. The combination of these three will produce a fruit of merit.
The seed, the planting of the seed, and the field are helpful to the givers and their departed relatives. The departed relatives experience happiness from the merit and the givers will receive more merit.
By doing wholesome deeds and sharing the merits that they receive with departed relatives, they are reborn in heaven to enjoy the results of their good deeds.
Read this translation of Petavatthu 1.1 Khettūpama Sutta: Like a Field by Ven.Kiribathgoda Gnananda Thero on SuttaFriends.org. Or explore the Pali on DigitalPaliReader.online.
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You can find the entire translation of the Petavatthu: Stories of Ghosts available on SuttaFriends.org.