Yes, I know that there's already a node about this, but this is a more accurate title.
Cube-shaped watermelons are yet another example frequently cited to prove that Those Japanese are crazy. They really do exist and are
made by simply putting a perfectly normal watermelon into a cubical container made of very strong glass and letting it grow in there. When the melon is fully grown, the container is shattered or disassembled and the melon is sold with a hefty price tag of about 10,000 yen, more than $80.
So why would someone go to such lengths to shape-change a fruit and why would people pay so much money for it (for, while all fruit is obscenely expensive in Japan, this is at least 3 times the price of a regular watermelon)? The reason usually cited in the look-at-those-crazy-Japanese-ha-ha-ha newspaper articles is that they fit better into refrigerators; after all, everyone knows that space is hard to come by in Japan. This is, of course, hogwash. It might have been the intention of the person who came up with it, but it simply would not be worth it, Japanese fridges aren't that small.
Cubical watermelons are made for one and only one reason: to be given as an original and expensive gift. Gift giving in Japan is considered very important, and a part of it is to discreetely ensure the recipient knows that the gift was expensive. Fruit in general is very popular, since everyone has a use for it, and expensive ready-made fruit gift boxes can be bought in many shops and supermarkets.
The cube-melons have received a lot of media attention, so everyone knows how expensive they are - an ideal gift with the added bonus of a curiosity factor.