It's a grape-sized fruit that grows on vines, also known as the Mexican sour gherkin or melothria scabra. It looks like a baby watermelon outside -- but more like a cucumber inside, as you can see in the picture above. It tastes like a cucumber, but a bit sweeter, maybe with a hint of lime -- nothing like a watermelon.
This plant is native to Mexico and Central America, and if it often eaten pickled. But you can eat it raw or lightly sautee it as well. Cucamelon recipes are quite healthy because they are packed with vitamins, minerals, antioxidants and fiber. And of course, they are low in calories, too.
About 135g, which equates to about 25-30 pieces or so, depending on size.