You gotta hand it to the Japanese.

When faced with a constraint, while the rest of the world tries to work around it, the
Japanese go for the jugular and find a solution for the problem. So, while the rest of
the world were content with watermelons in the way nature had designed it, these
guys found that the round shape was a problem when it came to stacking the fruit.

So, what did they do about it ? They engineered water-melons that are boxy in shape
and thus easy to stack. See picture below from the Deccan Chronicle of watermelons
that are boxy in shape.

That's ingenuity at work.