How to separate two jammed-together measuring cups? I have two Pyrex measuring jugs (this type) that have got jammed together, one inside the other, I'm looking for suggestions on how to separate them again.

I've already tried


washing-up liquid around the edge 
putting ice water in the upper jug, then dipping the bottom jug in hot water


but neither of these has worked.

What else can I do to separate them?
Filling the lower jug with water and then putting the whole thing in the freezer worked. I checked it after a couple of hours and didn't think it had worked as I still couldn't separate them, but it mustn't have finished freezing at that point because when I looked again a bit later the ice had travelled up between the two jugs and then at some point the inner jug must have popped free.