Porcelain bowls stuck together

[pennycarbonara]

Mystifying problem. Two heavy porcelain bowls I've had for years have got stuck together, one inside the other. They were stacked in the cupboard as usual, I got them out one day, and found they were jammed together, at a slight angle. Have tried washing them, leaving them in water, putting cooking oil in them to grease them, and they won't budge.
No idea why this has happened now.

Any ideas what to do about it?

[JohnLapsleyParlabane]

Bottom bowl in hot water and cold water with ice in the top one?
Washing up liquid in the join?

[LaChatte]

Try putting them in a warm oven (100°C) then lifting out top one with a tea towel?

[FlukeSkyeRunner]

Can you squirt wd40 in the join?

[Mapoftheworlddoorknob]

Drop them.