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https://en.wikipedia.org/wiki/Seedless_fruit will give you a technical explanation. Suffice to say that seedless fruit come from a parent plant that is not fertilized or is a sterile hybrid which is created by crossing male pollen for a watermelon, containing 22 chromosomes per cell, with a female watermelon flower with 44 chromosomes per cell. When this seeded fruit matures, the small, white seed coats inside contain 33 chromosomes, rendering it sterile and incapable of producing seeds. This is similar to the mule, produced by naturally crossing a horse with a donkey. This process does not involve genetic modification.