The main ingredient in a key lime pie is key lime juice, derived from the small, round key limes that are smaller and more aromatic than the more common Persian limes. The unique tartness of key lime juice is what gives the pie its distinctive flavor. Typically, key lime pie is made with a graham cracker crust and a filling consisting of sweetened condensed milk, egg yolks, and key lime juice, topped with whipped cream or meringue. This dessert is a classic dish from the Florida Keys, where key limes are traditionally grown, imbuing the pie with a local flavor.