You can buy square watermelons in Japan (developed to stack better in supermarkets).
Let's get one thing out of the way right up front: It doesn't matter what shape the watermelon is, it's what's inside that matters. That said, you have to admit there's something cool about watermelons that defy convention by assuming different shapes.
First, how. It's actually pretty easy (relatively speaking) to grow a square watermelon. Just about anyone can do it. While the watermelon is still small on the vine, a square, tempered glass box is placed around it. When the watermelon gets bigger, it assumes the shape of the box! You can do this too. There are even websites dedicated to teaching you how. Just remember to use a glass or transparent mold so the sunlight can reach the watermelon on all sides (except the bottom).
And why are square watermelons grown? Two reasons actually. First, the square watermelons are easier to stack, which makes them easier to ship. Second, and perhaps most ingeniously, with space being an issue in crowded areas of Japan, the square watermelon is designed to fit perfectly inside smaller Japanese refrigerators.
But, just like the heart-shaped creations, square watermelons cost a bit more than one shaped by Mother Nature.