In a medium bowl, combine the graham cracker crumbs, sugar, and melted butter. Mix well until all the crumbs are moistened. The mixture should resemble wet sand.

Press the graham cracker mixture evenly into the bottom and up the sides of a 9-inch pie plate. You can use the bottom of a measuring cup to help pack it down firmly.

For a firmer crust, bake in a preheated oven at 350°F (175°C) for 8-10 minutes. Let it cool completely before adding the filling. If you prefer a no-bake version, simply chill the crust in the refrigerator for at least 30 minutes.

In a large bowl, whisk together the sweetened condensed milk, key lime juice, and sour cream until smooth and well combined. The mixture will thicken slightly as you whisk.

Pour the key lime filling into the prepared graham cracker crust, spreading it evenly.

Cover the pie with plastic wrap (make sure the plastic wrap doesn't touch the filling) and refrigerate for at least 4 hours, or preferably overnight, to allow the filling to set completely.

Before serving, garnish with lime slices, if desired. Slice and enjoy!