Watermelon
Why: It’s low in calories (only 92 calories per 2-cup serving) but it fills you up; it’s not too expensive; and because it’s 92 percent water, it can quench your thirst and rehydrate your body on a hot summer day. The writer Mark Twain said the following about watermelon: "When one has tasted it, he knows what the angels eat."

Health Perks: On average, watermelon has 7.5 to 10 milligrams of the antioxidant lycopene per cup — that’s about 40 percent more than is found in raw tomatoes. That’s because lycopene’s bioavailability is low when tomatoes are eaten raw, but it increases in heat-processed products such as tomato juices and sauces. Watermelons, however, don’t need to be heated in order to obtain maximum lycopene, which is believed to guard against heart disease and some cancers.

Plus, watermelon contains citrulline, an amino acid that creates arginine, another amino acid that recent research suggests may help reduce the risk of cardiovascular disease, stimulate the immune system and accelerate the healing of wounds.

Additionally, watermelon is a good source of vitamins A (1 cup has almost 20 percent of your daily values) and C (20 percent of daily values), and it also contains potassium, vitamin B6 and thiamin.
Nutrition: Serving size: 1 cup, diced (152g), calories 46, total fat 0.2g, cholesterol 0mg, sodium 2mg, total carbohydrate 11.5g, dietary fiber 0.6g, sugars 9.4g, protein 0.9g.

Seek Out: Firm, juicy, red flesh without white streaks and skin without cracks, bruises or mold. The seeds should be dark brown or black. According to James Parker, associate global produce coordinator for Whole Foods Market, "Look for melons that have a uniform shape (not small on one end and larger on the other). Ripe fruit will have a slight give on the end opposite to the stem, and most varieties will also have a slight yellowing of the rind on the lightest part of the outside of the melon. ‘Thumping’ a melon is not at all reliable. The best way to tell for sure that it’s ripe is to have your grocer cut a plug out of the fruit for you (a small triangle that will show you how the inside looks)."

However, some experts rely on the "thumping method" because oftentimes you won’t have the opportunity to cut a plug and peek inside. Peter Romano, the produce director at New York City's famed Fairway Market, says that when you tap the melon in the middle with the palm of your hand, you should hear a hollow sound. Also, Romano suggests that if the watermelon is cut, it should have a thin rind and a bright, fire-engine-red color.

Avoid: If you have a chance to peek inside, (if the melon is sliced or if a plug is cut) avoid pale flesh, white streaks and whitish seeds. "And make sure there are no splits, veins, hollow pockets, dark red streaks or blood-red color (as opposed to an engine red color)," says Romano. The rind should be free of bruises, soft spots or mold. Also, avoid watermelons that are very hard, white or very pale green in color on the underside.

Storage: After it’s picked, watermelon doesn’t ripen very well. When unripe, keep it at room temperature. You can even try putting it in a large brown paper bag for a couple of days. According to chef and food expert Aliza Green, author of Field Guide to Produce (Quirk Books, 2004), ripe watermelon will keep best if cut up, covered with plastic wrap and refrigerated — it will last for about five days.

Interesting: Watermelon can also be considered a vegetable; it’s a relative of the pumpkin and squash family.

CHARLES STUART PLATKIN is a nutrition and public health advocate, author of The Diet Detective's Count Down (Simon & Schuster, 2007) and founder of DietDetective.com, the health and fitness network.