For safety reasons, heating the unopened can (an old cooking method) is NOT recommended.
Pour sweetened condensed milk into 9-inch pie plate. Cover with aluminum foil; place in larger shallow pan. Fill larger pan with hot water. Bake at 425 degrees F for 1 hour or until thick and caramel-colored. Beat until smooth.
Pour sweetened condensed milk into top of double boiler; place over boiling water. Simmer over low heat for 1 to 1/2 hours or until thick and caramel-colored, stirring occasionally. Beat until smooth.
Pour sweetened condensed milk into a 2-quart glass measuring cup. Cook on 50% power (medium) 4 minutes, stirring briskly every 2 minutes until smooth. Cook on 30% power (medium-low) 20 to 25 minutes or until very thick and caramel-colored, stirring briskly every 4 minutes during the first 16 minutes and every 2 minutes during the last 4 to 10 minutes.