Heat and cold. You need to make the outer dish expand and the inner dish contract (as much as they will, given that they are designed NOT to expand or contract!) I suggest that you put the outer dish into a sink or bowl of very hot water and put iced water in the inner dish. Leave for about 5 minutes, and then try again to separate them. You might need to dry them with cloth or paper towel.
If that doesn't work, then there is a really messy thing you could try: try to persuade some "penetrating and easing oil" (sold by hardware and bicycle stores, or "WD40") to ease the sticking point.
Margaret (SmithWitch), November 2007
