When I was a baby, my mom used to sing me a song about the moon eating a prickly-pear cactus fruit, and throwing the skins into a lagoon. Ahí viene la luna comiendo la tuna tirando las cáscaras a la laguna! (Note: after the word "laguna," you’re supposed to tickle the baby’s stomach.) Growing up, I knew the word "tuna" meant prickly-pear cactus fruit. But I had no desire to try them, because the idea of eating a cactus fruit seemed too weird. Even when I started to get more serious about food, I ignored them. Funny what moving to Mexico does -- this summer, surrounded by an abundance of tunas because of the rainy season, I realized that I'd misjudged them. Tunas are some of the juiciest, most naturally sweet fruits around. They have the wet, porous flesh of a watermelon, speckled with tiny hard seeds. In fact, an agua de tuna -- the juice of the tuna, mixed with water and sugar -- is one of the sweetest aguas frescas. Sometimes the drink can verge on cloying. Yesterday at the tianguis, I found a vendor who was practically giving his tunas away. His sign said as much: I bought a kilo, peeled... ...and decided to make an agua fresca de tuna that's less sweet than the ones I've tried. Mine would have lime and chia seeds to tone things down. Well, actually, the chia wouldn't really affect the sweetness factor, but it would add a healthy boost. The agua turned out even better than I hoped. It was a pretty pistachio color, and the taste was kind of like a melon-lemonade. I drank a glass after finishing up a tennis game (I'm taking classes, so this was my first game ever), and I couldn't have asked for a more refreshing drink. This agua was way better than Gatorade. Recipe below. Agua fresca de tuna (prickly-pear cactus fruit) with lime and chia Makes about 1 1/2 liters Ingredients 9 pieces of tuna fruit, peeled 1 tablespoon sugar water juice of three limes 2 tablespoons chia seeds Cut your tuna fruit into chunks. They don't have to be very small, because you're going to pop 'em in the blender anyway. If you're lucky enough to own a Vitamix or another extra-strong blender, you can toss them in the blender jar whole. Working in batches as to not overwhelm your blender, add your fruit chunks to the blender jar and pour just enough water to cover. On the first batch, add the two tablespoons of sugar. Blend until smooth. Strain into a pitcher using a fine-mesh strainer. Repeat with the remaining fruit, and add the lime juice on the last turn. At the very end, when you've got your pretty pitcher of green juice, stir in the chia seeds. The drink is fine at room temperature, but it's much more refreshing when it's served cold.