In a 10-inch skillet, cook the ground beef over medium-high heat for 6 to 8 minutes or until no longer pink, stirring to break up meat. Drain if necessary.
Stir in the bell pepper, taco seasoning mix, and water. Cook for 8 to 10 minutes or until the mixture thickens and the water is evaporated, stirring occasionally.
Meanwhile, heat the taco shells as directed on package.
Spoon 2 slightly rounded tablespoons of the beef mixture in each taco shell. Top with lettuce, tomato, and cheese. Garnish each taco with the sour cream.