The ancestor of the Big Gulp may be the huge Styrofoam cups of agua fresca served up in taquerias. Traditionally, the ice-cold drinks are ladled out of large glass jars. Economical, nutritious and fresh, agua fresca also comes together quickly in the home kitchen. Cut up some appropriate summer fruit—cantaloupe, watermelon, pineapple, mango or strawberries work well—and purée it in the blender until you have 3 cups or so. Add 1/2 cup of simple syrup. You make simple syrup by boiling two parts sugar and one part water for five minutes and then letting it cool. The syrup will keep for about three weeks in the fridge and will come in handy at the cocktail hour. Anyway, back to the recipe. Add 3 cups of water. Give this mixture a quick whirl, taste it and adjust the water and syrup to your liking. Pour it into a large glass filled with ice and take big gulps.