How do you select a ripe watermelon in the supermarket?
Try piercing the skin of the melon with your fingernail. If it is easy to do, the melon is most likely ripe. Old-timers swear they can tell if a melon is ripe simply by giving it a sharp rap. If thumping produces a dull, hollow thud, the melon is ripe. A higher, ringing sound means the fruit is still green.