People were talking about sweetened condensed milk, so I needed dulce de leche. Usually I just poke a hole in the lid of a can of sweetened condensed milk and boil it for a few hours, but I've started using the crock pot for things that boil over or away. And I always worried about the can exploding even though I poked that hole in. And I never knew if it was really done.
Anyway, this time I dumped the milk into a pint jar (actually, I did two cans in two jars) and covered it with foil and stuck it in the crock pot. Then I filled the pot with water till it came above the level of the milk in the jars, but below the level of the top of the jars. It took 6 hours on high and the glass jars made it easy to see when it was ready.
I've said before that I've never done much with my crock pot, but when I finally started using for things like this, my stove got a lot cleaner. I don't think I've ever forgotten something and boiled away all the water and had it burn, but I always, always have things boil over when I'm cooking stock or beans or milk for yogurt. The crock pot is perfect for things like that and I'm finally using the thing more than once a month.