How do you separate 2 stainless steel bowls that have stuck together?

This method is safest with ceramics or plastics, and may not be advisable with glass or other easily-breakable materials.

If the plate is stuck inside the bowl, fold a thick, plush towel in four on top of a counter or other hard surface. Hold the bowl by its base and turn it upside down (so the bowl's and plate's eating surfaces are facing the towel). Making sure your fingers won't get smashed, firmly but carefully bring the bowl-plate combo down onto the towel. Hopefully the plate will move within the bowl, or even come out, but it may take multiple strikes against the towel for the plate to come free. Be patient: don't try to do it all in one strike of the bowl-plate combo!

This method works because, when the bowl stops when it hits the towel, the plate still has momentum so it keeps moving.