Cut the top of each pita and open so you have a pocket. Mix yogurt, mint and sugar in a small bowl. Add cucumber, set aside. Mix meat, water, lemon juice, salt, cumin, oregano, pepper, garlic and onion. Shape mixture into four thin patties. Heat oil in a 10-inch skillet over medium heat. Cook patties in oil 10 - 12 minutes, until no longer pink in the center. Place cooked patty in each pita. Top with yogurt mixture, lettuce and tomato.