1. Preheat oven to 350°F. Pour the sweetened condensed milk in a pie dish. Place the pie dish in a large roasting pan and place the roasting pan in the oven. Fill the roasting pan with enough HOT water until it reaches halfway up the pie dish. Loosely cover the pie dish with aluminum foil.
2. Bake until sweetened condensed milk has thickened and caramelized, about 2 hours. Add more hot water to the roasting pan about half way through, since some of it will evaporate.