My daughter was packing ceramic canisters to bring to me and put one canister inside the other. I am unable to get them apart. Has anyone got any ideas?

Try placing the stack in warm/hot water. The outer one may expand enough to release. Could also fill the inner one with ice to increase the differential.

Before doing that, I would try submerging a small amount of dishwashing liquid and water.

One of my other thoughts was WD-40.

I tried the hot water and filled the inside of the top canister with ice. It didn't seem to be working so my husband tapped the outer canister with a rubber hammer and it finally let go. Thanks to all who offered advice.

Placing a paper towel or sheet of paper between glass or ceramic items when nesting for shipping avoids this problem and also prevents scratches and chipping from the items rubbing together during transport.