My first rule is to never buy a watermelon from the grocery store! Watermelons don't get ripe if picked green, or at least they don't get very sweet. I grew up eating watermelons bought for a quarter or so from the side of the road on the way home from Gulf Shores, still warm from the fields and sweet as candy. Luckily for me, people here in the Ozarks drive down to Louisiana or Mississippi and buy a trailer load and bring them up here to sell on the roadside. Can't get one for a quarter any longer, but Grandboy Joshua loves watermelon as I do, and he knows "We only buy watermelons on the side of the road!"