A co-worker of mine told me this and I didn't believe her.
You take a can of condensed milk and remove the label. Put it in a large/tall pot of water and slowly bring to a steady/low boil. Let it boil for 4 hours, remembering to add water when needed to keep the water level well above the top of the can. That step is very important because it could explode and really hurt you if you don't. Anyway, you let it boil for 4 hours and when it's done, carefully remove it from the water with tongs. Place it somewhere where it can cool, preferably with a paper towel beneath it to catch the water drips. Once it's cool enough to handle you can open it