Polyploidy and seedlessness
Firstly I can't tell you exactly why the water mellons are crying, maybe its about seedless babies, maybe its about not having babies, I don't know.
What I can tell you is why bananas are seedless and how seedless water mellons are made. Its not genetic enginering in the "insert genes from something else, now my strawberries glow in the dark" kind of way. Seedless fruit is caused by having an od number of sets of chromosomes. This causes the division of cells into gamestes (unfertilized eggs or spermies) to fail. These failed gametes are the little balck specks in bananas and white shrived things in seedless water mellons. Polyploidy is naturally occuring, but we humans are the ones who cultivate it.
For more information, consult your nearest biologist, or Wikipedia.
That said, I really want this shirt. Congrats to the designer.