Was trying to save room in cabinets, stacked a Glass 'glass' into a larger plastic 'glass' and it's stuck !!
How to get apart without breaking is the Q i guess.
Put the bottom glass in very hot water. Fill the inner glass with ice water. Wait a few minutes. The outside glass should expand from the heat and the inner glass should contract from the cold. you should be able to separate them.
Thanks, I'll try that and get back.