Open the cans of condensed milk and pour them evenly into the jars. One jar will fill three 4 ounce canning jars perfectly. Leave a little room for expansion.
Put the jars into the crock of your slow cooker.
Add water to cover the jars by at least an inch.
Cook on low for 10 hours, or until the caramel is the color you want it to be.
Carefully remove the jars, and let cool completely before opening.
