Popsicles that are prickly? Well, prickly as in 'prickly pear' juice from the cactus fruit that ripens into a sweet, amethyst treat in midsummer in the deserts of the Southwest. They're called tuna in Spanish. Stop by the Farmers' Market on a Sunday morning in Tucson, and check out the cactus juice popsicles, displayed on chunks of dry ice. The challenge is to eat it before the juice melts down to your elbows.