Wash the nail and pound it into the top of can of sweetened condensed milk to make 2 small holes on opposite sides of the top. Remove nail.
Place the can of condensed milk into a small saucepan and pour in water to reach high up the side of the can. Bring water to a boil; reduce heat to low. Simmer milk in the can for 3 to 4 hours. Milk will slowly turn light to medium brown and become thicker as it cooks. Add more water as it boils down.
Remove lid from can and pour dulce de leche into a bowl. Whisk to break up any lumps and mix thicker caramel from the bottom into the rest of the syrup.
