Super Simple Key Lime Pie

1 can sweetened condensed milk
1/2 c. key lime juice (best if fresh, but bottled works too)
1 tsp. key lime zest (if using bottled key lime juice, you can use a regular lime for the zest)
1 (8 oz.) container Cool Whip
9" pie shell, baked (graham cracker crust works too, but it's really really sweet)

Mix milk, juice and zest. Fold in Cool Whip till well blended. Color if desired (I like a barely yellow-green tint). Cool in fridge till stiffened (at least 1 hour). Garnish with Cool Whip and key lime slices or zest.