system
Given the following post, respond as if you're commenting on Reddit:
human
How do you grow "seedless" watermelons?
gpt
You use a chemical called colchicine, which prevents cells from dividing by only letting their nucleus divide, not the citoplasm. It's called karyokenisis without cytokenisis. This leads to a tetraploid, an organism with four sets of chromosomes.

Plants are quite a bit more flexible than animals. They can survive with two, three, four, or even eight sets of chromosomes and still be able to grow.

This tetraploid plant is then crossed with a diploid, which produces triploid. The total of two chromosomes from one parent plus one chromosome from the other makes 3 chromosomes for the offspring. 

However, because these plants don't have even pairs of chromosomes they cannot undergo successful meiosis because it requires chromosomes to be split into two even sets to create viable gamets. So they're infertile.

But plants have a different way of doing things to animals. Male gamets have 2 different nuclei, which do different things, even triggering the creation of fruit within a flower without the need for viable eggs. Even when two gamets fuse, many different kinds of nuclei are formed, all of which do different things such as create the endosperm, others are thought to tell the pant to know which way is up, etc. Since watermelon triploids produce flowers, they can be polinised by other plants, farmers allow them to be polinated by diploids. This triggers watermelons to grow, but since there are no viable eggs within the flower, seeds can't form.

This is also how many other seedless fruits are made. It's also important to note that polyploidy makes fruits bigger because cells have to accommodate more stuff. Strawberries are octoploids, and if i remember correctly apples are tetraploids. Furthermore, polyploidies in plants can also come about naturally or by selective pressures and even-numbered polyploidies often produce viable gamets.