To understand why, you need to understand its nature. A watermelon’s sweetness is determined largely by the weather, the drier during harvest the better. Donald Sherman, 63, a third-generation watermelon farmer in Fresno, Calif., explains that watermelons don’t gain sweetness once cut off the vine. “If they don’t have that sugar out with Mother Nature, they’re not getting more.”

Fruits like peaches and pears will sweeten as they ripen, but, according to Juan Anciso, a professor and vegetable specialist with Texas A&M AgriLife Extension Service, harvested watermelons continue to ripen only “in the sense that they soften and start to decay.”

So you’re not scouting potential when looking for peak melon. And before you look, you want to listen. (There’s nothing to smell.) Both Sherman and Anciso said that sound is the most important factor in choosing a watermelon, and that the best way to hear it is with an openhanded smack.

You’re listening for tone, not volume. Anciso compares the full sound of a good melon to “beating on a drum.” When you slap it, the sound should bounce back. If the sound is absorbed, that means the inside is old and mealy. Sherman likens an overripe tone to an overly flat sound, like slapping a flat basketball.

Pitch is important, too. Sherman suggests slapping a number of melons in a bin to hear the differences between the “tighter, higher pitch” and the ones with low bass notes. “If you find something in between, that’s the good-quality melon,” he said. At the beginning of the season and in high summer, the best ones have a deeper sound. In September, those with slightly higher pitches are better.

Sherman said that shift in sound is the result of the watermelons having more dew on their vines, a nuance he’s learned over the years. Another nuance: the ability to spot good melons by their color, which lightens when ripe. He suggests looking for paler ones.

Anciso recommends finding melons with “yellow bellies.” Most watermelons grown in the United States sit on the soil, and the spot where they rest develops a patch that starts white then turns creamy yellow once the fruit has matured.

Once you’ve found just the right one, cut into it right away because you don’t want it to become overripe and mealy. If you choose one that’s not good enough to eat on its own, that’s all right. A little salt on sort-of-bland slices can heighten the sweetness. And melon that really needs a boost can become sugary as Popsicles, boozy blended into a margarita, savory in a salad with feta, and showstopping in a creamy trifle.