Experts are studying whether the temperature of the watermelon when eaten -- cold or chilled vs. room temperature -- affects the nutritional value. Some studies have shown that cold melon may have fewer vitamins, but more research is needed.

Fans of the controversial glycemic index might know that watermelon has a high ("bad") GI rating. However, its glycemic load, which may be a more useful measure because it accounts for the carb content in a serving and the effect on blood sugar, is much lower.

Types of watermelon

Watermelons have more than 50 varieties and come in numerous sizes and shapes. Cut one open and you may find yellow or orange meat or the traditional red. There are seedless varieties for those who don't want to ungraciously spit out seeds at the dinner table. Some are oblong-shaped, and others are more like basketballs. They have even been grown into cubes.

Some varieties grown in Central and South America are small (about the size of a large grapefruit) with thinner rinds, softer skins and few seeds. The largest watermelon, according to the Guinness Book of World Records, weighed in at a whopping 262 pounds!

How to pick a good watermelon

Selecting the best possible choice for a watermelon isn't that hard. You want to choose one that is firm and has no dents, cuts or bruises. A good watermelon will feel heavy, because of the high water content.

Lastly, look for a ground spot. This is where the watermelon rested on the ground as it grew and ripened on the vine. The spot should be yellow and creamy in color.